C APACITY OF G ROUP - INVARIANT L INEAR R EADOUTS FROM E QUIVARIANT R EPRESENTATIONS : H OW M ANY O BJECTS CAN BE L INEARLY C LASSIFIED U NDER A LL P OSSIBLE V IEWS ?
Anonymous authors
Paper under double-blind review
A BSTRACT
Equivariance has emerged as a desirable property of representations of objects subject to identity-preserving transformations that constitute a group, such as translations and rotations.
However, the expressivity of a representation constrained by group equivariance is still not fully understood.
We address this gap by providing a generalization of Cover’s Function Counting Theorem that quantiﬁes the number of linearly separable and group-invariant binary dichotomies that can be assigned to equivariant representations of objects.
We ﬁnd that the fraction of separable dichotomies is determined by the dimension of the space that is ﬁxed by the group action.
We show how this relation extends to operations such as convolutions, element-wise nonlinearities, and global and local pooling.
While other operations do not change the fraction of separable dichotomies, local pooling decreases the fraction, despite being a highly nonlinear operation.
Finally, we test our theory on intermediate representations of randomly initialized and fully trained convolutional neural networks and ﬁnd perfect agreement.
I NTRODUCTION
The ability to robustly categorize objects under conditions and transformations that preserve the object categories is essential to animal intelligence, and to pursuits of practical importance such as improving computer vision systems.
However, for general-purpose understanding and geometric reasoning, invariant representations of these objects in sensory processing circuits are not enough.
Perceptual representations must also accurately encode their transformation properties.
One such property is that of exhibiting equivariance to transformations of the object.
When such transformations are restricted to be an algebraic group, the resulting equivariant representations have found signiﬁcant success in machine learning starting with classical convolutional neural networks (CNNs) (Denker et al., 1989; LeCun et al., 1989) and recently being generalized by the inﬂuential work of Cohen & Welling (2016).
Such representations have elicited burgeoning interest as they capture many transformations of practical interest such as translations, permutations, rotations, and reﬂections.
Furthermore, equivariance to these transformations can be easily “hard-coded” into neural networks.
Indeed, a new breed of CNN architectures that explicitly account for such transformations are seeing diverse and rapidly growing applications (Townshend et al., 2021;
Baek et al., 2021;
Satorras et al., 2021; Anderson et al., 2019; Bogatskiy et al., 2020; Klicpera et al., 2020;
Winkels & Cohen, 2019; Gordon et al., 2020; Sosnovik et al., 2021; Eismann et al., 2020).
In addition, equivariant CNNs have been shown to capture response properties of neurons in the primary visual cortex beyond classical G´abor ﬁlter models (Ecker et al., 2018).
While it is clear that equivariance imposes a strong constraint on the geometry of representations and thus of perceptual manifolds (Seung & Lee, 2000; DiCarlo & Cox, 2007) that are carved out by these representations as the objects transform, the implications of such constraints on their expressivity are not well understood.
In this work we take a step towards addressing this gap.
Our starting point is the classical notion of the perceptron capacity (sometimes also known as the fractional memory/storage capacity) – a quantity fundamental to the task of object categorization and closely related to VC dimension (Vapnik & Chervonenkis, 1968).
Deﬁned as the maximum number of points for which all (or 1- δ fraction of) possible binary label assignments (i.e. dichotomies ) afford a hyperplane that separates points with one label from the points with the other, it can be seen to offer a quantiﬁcation of the expressivity of a representation.
Classical work on perceptron capacity focused on points in general position (Wendel, 1962; Cover, 1965; Schl¨aﬂi, 1950; Gardner, 1987; 1988).
However, understanding the perceptron capacity when the inputs are not merely points, but are endowed with richer structure, has only recently started to attract attention.
For instance, work by Chung et al.
; Pastore et al.
(2020); Rotondo et al.
; Cohen et al.
(2020) considered general perceptual manifolds and examined the role of their geometry to obtain extensions to the perceptron capacity results.
However, such work crucially relied on the assumption that each manifold is oriented randomly, a condition which is strongly violated by equivariant representations.
With these motivations, our particular contributions in this paper are the following:
• We extend Cover’s function counting theorem and VC dimension to equivariant representations, ﬁnding that both scale with the dimension of the subspace ﬁxed by the group action.
• We demonstrate the applicability of our result to G -convolutional network layers, including pooling layers, through theory and verify through simulation.
1. R ELATED WORKS
Work most related to ours falls along two major axes.
The ﬁrst follows the classical perceptron capacity result on the linear separability of points (Schl ¨ aﬂi, 1950; Wendel, 1962; Cover, 1965;
Gardner, 1987; 1988).
This result initiated a long history of investigation in theoretical neuroscience, e.g. (Brunel et al., 2004; Chapeton et al., 2012;
Rigotti et al., 2013; Brunel, 2016; Rubin et al., 2017; Pehlevan & Sengupta, 2017), where it is used to understand the memory capacity of neuronal architectures.
Similarly, in machine learning, the perceptron capacity and its variants, including notions for multilayer perceptrons, have been fundamental to a fruitful line of study in the context of ﬁnite sample expressivity and generalization (Baum, 1988; Kowalczyk, 1997; Sontag, 1997; Huang, 2003; Yun et al., 2019; Vershynin, 2020).
Work closest in spirit to ours comes from theoretical neuroscience and statistical physics (Chung et al., 2018;
Pastore et al., 2020; Rotondo et al., 2020;
Cohen et al., 2020), which considered general perceptual manifolds, albeit oriented randomly, and examined the role of their geometry to obtain extensions to the perceptron capacity result.
The second line of relevant literature is that on group-equivariant convolutional neural networks (GCNNs).
The main inspiration for such networks comes from the spectacular success of classical
CNNs (LeCun et al., 1989) which directly built in translational symmetry into the network architecture.
In particular, the internal representations of a CNN are approximately 1 translation equivariant: if the input image is translated by an amount t , the feature map of each internal layer is translated by the same amount.
Furthermore, an invariant read-out on top ensures that a CNN is translation invariant.
Cohen & Welling (2016) observed that a viable approach to generalize CNNs to other data types could involve considering equivariance to more general transformation groups.
This idea has been used to construct networks equivariant to a wide variety of transformations such as planar rotations (Worrall et al., 2017; Weiler et al., 2018b;
Bekkers et al., 2018; Veeling et al., 2018;
Smets et al., 2020), 3D rotations (Cohen et al., 2018; Esteves et al., 2018; Worrall & Brostow, 2018;
Weiler et al., 2018a; Kondor et al., 2018a; Perraudin et al., 2019), permutations (Zaheer et al., 2017;
Hartford et al., 2018; Kondor et al., 2018b; Maron et al., 2019a; 2020), general Euclidean isometries (Weiler et al., 2018a; Weiler & Cesa, 2019; Finzi et al., 2020), scaling (Marcos et al., 2018;
Worrall & Welling, 2019; Sosnovik et al., 2020) and more exotic symmetries (Bogatskiy et al., 2020;
Shutty & Wierzynski, 2020; Finzi et al., 2021) etc.
A quite general theory of equivariant/invariant networks has also emerged.
Kondor & Trivedi (2018) gave a complete description of GCNNs for scalar ﬁelds on homogeneous spaces of compact groups.
This was generalized further to cover the steerable case in (Cohen et al., 2019b) and to general gauge ﬁelds in (Cohen et al., 2019a; Weiler et al., 2021).
This theory also includes universal approximation results (Yarotsky, 2018; Keriven & Peyr´e, 2019; Sannai et al., 2019b; Maron et al., 2019b; Segol & Lipman, 2020; Ravanbakhsh, 2020).
Nevertheless, while beneﬁts of equivariance/invariance in terms of improved sample complexity and ease of training are quoted frequently, a ﬁrm theoretical understanding is still largely missing.
Some results however do exist, going back to (Shawe-Taylor, 1991).
Abu-Mostafa (1993) made the argument that restricting a classiﬁer to be invariant can not increase its VC dimension.
Sokolic et al.
(2017) extend this idea to derive generalization bounds for invariant classiﬁers, while Sannai et al.
(2019a) do so speciﬁcally working with the permutation group.
Elesedy & Zaidi (2021) show a strict generalization beneﬁt for equivariant linear models, showing that the generalization gap between a least squares model and its equivariant version depends on the dimension of the space of anti-symmetric linear maps.
Some beneﬁts of related ideas such as data augmentation and invariant averaging are formally shown in (Lyle et al., 2020; Chen et al., 2020).
Here we focus on the limits to expressivity enforced by equivariance.
P ROBLEM FORMULATION
Suppose x abstractly represents an object and let r ( x ) ∈ R N be some feature map of x to an
N -dimensional space (such as an intermediate layer of a deep neural network).
We consider transformations of this object, such that they form a group in the algebraic sense of the word.
We denote the abstract transformation of x by element g ∈ G as g x .
Groups G may be represented by invertible matrices, which act on a vector space V (which themselves form the group GL ( V ) of invertible linear transformations on V ).
We are interested in feature maps r which satisfy the following group equivariance condition:
[Equation]
where π : G → GL ( R N ) is a linear representation of G which acts on feature map r ( x ) .
Note that many representations of G are possible, including the trivial representation: π ( g )
=
I for all g .
We are interested in perceptual object manifolds generated by the actions of G .
Each of the P manifolds can be written as a set of points { π ( g ) r µ : g ∈ G } where µ ∈ [ P ] ≡ { 1 , 2 , . . .
, P } ; that is, these manifolds are orbits of the point r µ ≡ r ( x µ ) under the action of π .
We will refer to such manifolds as π -manifolds .
Each of these π -manifolds represents a single object under the transformation encoded by π ; hence, each of the points in a π -manifold is assigned the same class label.
A perceptron endowed with a set of linear readout weights w will attempt to determine the correct class of every point in every manifold.
The condition for realizing (i.e. linearly separating) the dichotomy { y µ } µ can be written as y µ w (cid:62) π ( g ) r µ > 0 for all g ∈ G and µ ∈ [ P ] , where y µ = +1 if the µ th manifold belongs to the ﬁrst class and y µ = − 1 if the µ th manifold belongs to the second class.
The perceptron capacity is the fraction of dichotomies that can be linearly separated; that is, separated by a hyperplane.
For concreteness, one might imagine that each of the r µ is the neural representation for an image of a dog (if y µ = +1 ) or of a cat (if y µ = − 1 ).
The action π ( g ) could, for instance, correspond to the image shifting to the left or right, where the size of the shift is given by g .
Different representations of even the same group can have different coding properties, an important point for investigating biological circuits and one that we leverage to construct a new GCNN architecture in Section 5.
P ERCEPTRON C APACITY OF G ROUP -G ENERATED M ANIFOLDS
Here we ﬁrst state and prove our results in the general case of representations of compact groups over R N .
The reader is encouraged to read the proofs, as they are relatively simple and provide valuable intuition.
We consider applications to speciﬁc group representations and GCNNs in Sections 4 and 5 that follow.
3. S EPARABILITY OF π - MANIFOLDS
We begin with a lemma which states that classifying the P π -manifolds can be reduced to the problem of classifying their P centroids.
For the rest of Section 3, we let π : G → GL( R N ) be an arbitrary linear representation of a compact group G .
We denote the average of π over G with respect to the Haar measure by (cid:104) π ( g ) (cid:105) g ∈ G ; for ﬁnite G this is simply 1 | G | (cid:80) g ∈ G π ( g ) where | G | is the order (i.e. number of elements) of G .
For ease of notation we will generally write (cid:104) π (cid:105) ≡ (cid:104)
π ( g ) (cid:105) g ∈ G when the group G being averaged over is clear.
Lemma 1.
A dataset { ( π ( g ) r µ , y µ ) } g ∈ G,µ
∈ [ P ] consisting of P π -manifolds with labels y µ is linearly separable if and only if the dataset { ( (cid:104) π (cid:105) r µ , y µ ) }
µ ∈ [ P ] consisting of the P centroids (cid:104) π (cid:105) r µ with the same labels is linearly separable.
Formally,
[Equation]
Proof.
The forward implication is obvious: if there exists a w which linearly separates the P manifolds according to an assignment of labels y µ , that same w must necessarily separate the centroids of these manifolds.
This can be seen by averaging each of the quantities y µ w (cid:62) π ( g ) r µ over g ∈ G .
Since each of these quantities is positive, the average must be positive.
For the reverse implication, suppose y µ w (cid:62) (cid:104) π (cid:105) r µ > 0 , and deﬁne ˜ w = (cid:104) π (cid:105) (cid:62) w .
We will show that ˜ w separates the P π -manfolds since
[Equation]
(Deﬁnition of ˜ w )
[Equation]
(Deﬁnition of (cid:104) π (cid:105) and linearity of π ( g )
[Equation]
=
[Equation]
Invariance of the Haar Measure µ ( Sg ) = µ [Equation]
S ) for set S ) >
(Assumption that w separates centroids)
Thus, all that is required to show that ˜ w separates the π -orbits are basic properties of a group representation and invariance of the Haar measure to G -transformations.
3. R ELATIONSHIP WITH C OVER ’ S T HEOREM AND VC DIMENSION
The fraction f of linearly separable dichotomies on a dataset of size P for datapoints in general position 4 in N dimensions was computed by Cover (1965) and takes the form
[Equation]
{+where we take (cid:0) nm (cid:1) = 
 for m > n . Taking α = P/N , f is a nonincreasing sigmoidal function+} of α that takes the value 1 when α ≤ 1 , 1/2 when α = 2 , and approaches 0 as α → ∞ .
In the thermodynamic limit of P, N → ∞ and α = O (1) , f ( α ) converges to a step function ¯ f ( α ) = lim P,N →∞ ,α = P/N f ( P, N ) =
Θ(2 − α ) (Gardner, 1987; 1988; Shcherbina & Tirozzi, 2003), indicating that in this limit all dichotomies are realized if α < 2 and no dichotomies can be realized if α > 2 , making α c = 2 a critical point in this limit.
The VC dimension (Vapnik & Chervonenkis, 1968) of the perceptron trained on points in dimension
N is deﬁned to be the largest number of points P such that all dichotomies are linearly separable for some choice of the P points. These points can be taken to be in general position. Because f ( P, N ) = 1 precisely when P ≤ N , the VC dimension is always N = P for any ﬁnite N and P (Abu-Mostafa et al., 2012), even while the asymptotics of f reveal that f ( P, N ) approaches 1 at any N > P/ 2 as N becomes large. In this sense the perceptron capacity yields strictly more information than the VC dimension, and becomes comparatively more descriptive of the expressivity as N becomes large.
In our G -invariant storage problem, the relevant scale is α = P/N 0 where N 0 is the dimension of the ﬁxed point subspace (deﬁned below).
We are now ready to state and prove our main theorem.
Theorem 1.
Suppose the points (cid:104) π (cid:105) r µ for µ ∈ [ P ] lie in general position in the subspace V
=
range( (cid:104) π (cid:105) )
= {(cid:104) π (cid:105) x : x ∈ V } . Then V 0 is the ﬁxed point subspace of π , and the fraction of linearly separable dichotomies on the P π -manifolds { π ( g ) r µ : g ∈ G } is f
( P, N 0 ) , where N 0 = dim V 0 .
Equivalently, N 0 is the number of trivial irreducible representations that appear in the decomposition of π into irreducible representations.
The ﬁxed point subspace is the subspace W = { w ∈ V | gw = w , ∀ g ∈ G } .
The theorem and its proof use the notion of irreducible representations (irreps), which are in essence the fundamental building blocks for general representations.
Concretely, any representation of a compact group over a real vector space decomposes into a direct sum of irreps (Naimark & Stern, 1982; Bump, 2004).
A representation π : G → GL( V ) is irreducible if V is not
and if no vector subspace of V is stable under G , other than 0 and V (which are always stable under G ).
A subspace W being stable (or invariant) under G means that π ( g ) w ∈ W for all w ∈ W and g ∈ G .
The condition that the points (cid:104) π (cid:105) r µ be in general position essentially means that there is no prescribed special relationship between the r µ and between the r µ and (cid:104) π (cid:105) .
Taking the r µ to be drawn from a full-rank Gaussian distribution is sufﬁcient to satisfy this condition.
Proof.
By the theorem of complete reducibility (see Fulton & Harris (2004)), π admits a decomposition into a direct sum of irreps π ∼ =
π k 1 ⊕ π k 2 ⊕ ...
⊕ π k M acting on vector space V =
V 1 ⊕ V 2 ⊕ ...
V M , where ∼ = denotes equality up to similarity transformation.
The indices k j indicate the type of irrep corresponding to invariant subspace V j .
The ﬁxed point subspace V 0 is the direct sum of subspaces where trivial k j = 0 irreps act: V
= (cid:76) n : k n =0 V n .
By the Grand Orthogonality Theorem of irreps (see Liboff (2004)) all non-trivial irreps average to zero (cid:104) π k,ij ( g ) (cid:105) g ∈ G ∝ δ k,
δ i,j .
Then, the matrix (cid:104) π (cid:105) simply projects the data to V 0 .
By Lemma 1 the fraction of separable dichotomies on the πmanifolds is the same as that of their centroids (cid:104) π (cid:105) r µ .
Since, by assumption, the P points (cid:104) π (cid:105) r µ lie in general position in V 0 , the fraction of separable dichotomies is f ( P, dim V 0 ) by Equation 1. Remark:
A main idea used in the proof is that only nontrivial irreps average to zero.
This will be illustrated with examples in Section 4 below.
Remark:
For ﬁnite groups, N 0 can be easily computed by averaging the trace of π , also known as the character , over G : N 0 = (cid:104)
Tr( π ( g ))
(cid:105) g ∈ G =
Tr( (cid:104) π (cid:105) )
(see Serre (2014)).
Remark:
If the perceptron readout has a bias term b i.e. the output of the perceptron is w (cid:62) π
( g ) r + b , then this can be thought of as adding an invariant dimension.
This is because the output can be written ˜ w (cid:62) ˜
π ( g )˜ r where ˜ w = ( w , b ) , ˜ r
= ( r , 1) , and ˜ π ( g ) =
π ( g ) ⊕ 1 is π with an extra row and column added with a 1 in the last position and zeros everywhere else.
Hence the fraction of separable dichotomies is f ( P, N 0 + 1) .
These results extend immediately to a notion of VC-dimension of group-invariant perceptrons.
Corollary 1.
Let the VC dimension for G -invariant perceptrons with representation π , N πVC , denote the maximum number P , such that there exist P anchor points { r µ } P µ =1 so that { ( π ( g ) r µ , y µ ) } µ ∈ [ P ] ,g ∈ G is separable for all possible binary dichotomies { y µ } µ ∈
[ P ] . Then N πVC = dim ( V 0 ) .
Proof. By theorem 1 and Equation 1, all possible dichotomies are realizable for P ≤ dim ( V 0 ) provided the points (cid:104) π (cid:105) r µ are in general position. Our theory also extends immediately to subgroups. For example, strided convolutions are equivariant to subgroups of the regular representation.
Corollary 2.
A solution to the the G -invariant classiﬁcation problem necessarily solves the G (cid:48) - invariant problem where G (cid:48) is a subgroup of G .
Proof.
Assume that y µ w (cid:62) r ( g x µ ) > 0 for g ∈ G .
G (cid:48) ⊆
G = ⇒ y
µ w (cid:62) r
( g x µ ) > 0 ∀ g ∈ G (cid:48) .
Consequently, the G (cid:48) -invariant capacity is always higher than the capacity for the G -invariant classiﬁcation problem.
[-Finally, we state the asymptotic capacity of a group-invariant perceptron by applying Gardner’s result (Gardner, 1987; 1988): Corollary 3.2.2. The G -invariant perceptron capacity for a π -equivariant code in the asymptotic limit N 0 , P → ∞ with α = P/N 
 =-][Figure]
E XAMPLE A PPLICATION : T HE CYCLIC GROUP Z m
In this section we illustrate the theory in the case of the cyclic group G =
Z m on m elements.
This group is isomorphic to the group of integers { 0 , 1 , ..., m − 1 } under addition modulo m , and this is the form of the group that we will consider.
An example of this group acting on an object is an image that is shifted pixel-wise to the left and right, with periodic boundaries.
In Appendix A.2 we show an application of our theory to the non-abelian Lie group SO (3) .
4. R OTATION MATRICES
The 2 × 2 discrete rotation matrices R ( θ g ) ≡ (cid:20) cos( θ g ) − sin( θ g ) sin( θ g ) cos( θ g ) (cid:21) where θ g = 2 πg/m and g ∈ Z m , are one possible representation of Z m ; in this case V = R 2 .
This representation is irreducible and nontrivial, which implies that the dimension of the ﬁxed point subspace is 0 (only the origin is mapped to itself by R for all g ).
Hence the fraction of linearly separable dichotomies of the π -manifolds by Theorem 1 is f ( P, 0) .
This result can be intuitively seen by plotting the orbits, as in Figure 1a for m = 4 .
Here it is apparent that it is impossible to linearly separate two or more manifolds with different class labels, and that the nontrivial irrep R averages to the zero matrix.
The representation can be augmented by appending trivial irreps, deﬁning π : G → GL( R N ) by π ( g )
=
R ( θ g ) ⊕ I ≡ (cid:20) R ( θ g )
I (cid:21) where I is an ( N − 2) × ( N − 2) -dimensional identity matrix.
The number of trivial irreps is N − 2 , so that the capacity is f ( P, N − 2) .
This is illustrated in
Figure 1b for the case N = 3 .
Here we can also see that the trivial irrep, which acts on the subspace span { (0 , 0 , 1) } , is the only irrep in the decomposition of π that does not average to zero.
This ﬁgure also makes intuitive the result of Lemma 1 that dichotomies are realizable on the π
-manifolds if and only if the dichotomies are realizable on the centroids of the manifolds.
4. T HE REGULAR REPRESENTATION OF Z m
Suppose π : Z m → GL( V ) is the representation of Z m consisting of the cyclic shift permutation matrices (this is called the regular representation of Z m ).
In this case V =
R m and π ( g ) is the matrix that cyclically shifts the entries of a length- m vector g places.
For instance, if m = 3 and v = (1 , 2 , 3) then π (2) v = (2 , 3 , 1) .
In Appendix A.1 we derive the irreps of this representation, which consist of rotation matrices of different frequencies.
There is one copy of the trivial irrep π 0 ( g ) ≡ 1 corresponding with the ﬁxed point subspace span { 1 m } where 1 m is the length- m all-ones vector.
Hence the fraction of separable dichotomies is f ( P, 1) .
This is illustrated in Figure 1c in the case where m = 3 .
The average of the regular representation matrix is (cid:104) π (cid:105) =
1 | G | 1 m 1 (cid:62) m , indicating that (cid:104) π (cid:105) projects data along 1 m .
D IRECT SUMS OF R EGULAR R EPRESENTATIONS
For our last example we deﬁne a representation using the isomorphism
Z m ∼ =
Z m 1 ⊕ Z m 2 for m = m 1 m
and m 1 and m 2 coprime 6 .
Let π (1) :
Z m 1 → GL( R m 1 ) and π (2) :
Z m 2 → GL( R m 2 ) be the cyclic shift representations (i.e. the regular representations) of Z m 1 and Z m 2 , respectively.
Consider the representation π (1) ⊕ π (2) :
Z m → GL( R m 1 + m 2 ) deﬁned by ( π (1) ⊕ π (2) )
( g ) ≡ π (1) ( g mod m 1 ) ⊕ π (2) ( g mod m 2 ) , the block-diagonal matrix with π (1) ( g mod m 1 ) being the ﬁrst and π (2) ( g mod m 2 ) the second block.
There are two copies of the trivial representation in the decomposition of π (1) ⊕ π (2) , corresponding to the one-dimensional subspaces span { ( 1 m 1 , 0 m 2 ) } and span { ( 0 m 1 , 1 m 2 ) } ,
where 0 m 1 is the length- k vector of all zeros.
Hence the fraction of separable dichotomies is f ( P, 2) .
This reasoning extends simply to direct sums of arbitrary length (cid:96) , yielding a fraction of f ( P, (cid:96) ) .
7 These representations are used to build and test a novel G-convolutional layer architecture with higher capacity than standard CNN layers in Section 5.
These representations are analogous to certain formulations of grid cell representations as found in entorhinal cortex of rats (Hafting et al., 2005), which have desirable qualities in comparison to place cell representations (Sreenivasan & Fiete, 2011).
Precisely, a collection { Z m k ×
Z m k } k of grid cell modules encodes a large 2-dimensional spatial domain Z m × Z m where m = (cid:81)
k m k .
G-E QUIVARIANT N EURAL N ETWORKS
The proposed theory can shed light on the feature spaces induced by G -CNNs.
Consider a single convolutional layer feature map for a ﬁnite 9 group G with the following activation
[Equation]
for some nonlinear function φ .
For each ﬁlter i , and under certain choices of π , the feature map a i ( x ) ∈ R | G | exhibits the equivariance property a i ( g k x )
=
π ( g k ) a i ( x ) .
We will let a ( x ) ∈ R | G | N denote a ﬂattened vector for this feature map.
In a traditional periodic convolutional layer applied to inputs of width W and length L , the feature map of a single ﬁlter a i ( x ) ∈ R | G | is equivariant with the regular representation of the group
G =
Z W × Z L (the representation that cyclically shifts the entries of W × L matrices).
Here the order of the group is | G | = WL .
Crucially, our theory shows that this representation contributes exactly one trivial irrep per ﬁlter (see Appendix A.3.1).
Since the dimension of the entire collection of N feature maps a ( x ) is N | G | for ﬁnite groups G , one might naively expect capacity to be P ∼ 2 N | G | for large N from Gardner (1987).
However, Theorem 1 shows that for G -invariant classiﬁcation, only the trivial irreps contribute to the classiﬁer capacity.
Since the number of trivial irreps present in the representation is equal to the number of ﬁlters N , we have P ∼ 2 N .
We show in Figure 2 that our prediction for f ( P, N ) matches that empirically measured by training logistic regression linear classiﬁers on the representation.
We perform this experiment on both (a) a random convolutional network and (b) VGG-11 (Simonyan & Zisserman, 2015) pretrained on CIFAR-10 (Krizhevsky, 2009) by (liukuang, 2017).
For these models we vary α by ﬁxing the number of input samples and varying the number of output channels by simply removing channels from the output tensor.
The convolutions in these networks are modiﬁed to have periodic boundary conditions while keeping the actual ﬁlters the same – see Appendix A.3.1 and Figure A.1 for more information and the result of using non-periodic convolutions, which impact the capacity but not the overall scaling with N 0 .
Other GCNN architectures can have different capacities.
For instance, a convolutional layer equivariant to the direct sum representation of Section 4.3 has double the capacity with P ∼ 4 N (Figure 2c), since each output channel contributes two trivial irreps.
See Appendix A.3.4 for an explicit construction of such a convolutional layer and a derivation of the capacity.
5. P OOLING O PERATIONS
In convolutional networks, local pooling is typically applied to the feature maps which result from each convolution layer.
In this section, we describe how our theory can be adapted for codes which contain such pooling operations.
We will ﬁrst assume that π is an N -dimensional representation of
G .
Let P ( r ) : R N → R N/k be a pooling operation which reduces the dimension of the feature map.
The condition that a given dichotomy { y µ } is linearly separable on a pooled code is
[Equation]
We will ﬁrst analyze the capacity when P ( · ) is a linear function (average pooling) before studying the more general case of local non-linear pooling on one and two-dimensional signals.
5.1. L OCAL A VERAGE P OOLING
In the case of average pooling, the pooling function P ( · ) is a linear map, represented with matrix
P which averages a collection of feature maps over local windows.
Using an argument similar to
Lemma 1 (Lemma 2 and Theorem 2 in Appendix A.3.2), we prove that the capacity of a standard CNN is not changed by local average pooling: for a network with N ﬁlters, local average pooling preserves the one trivial dimension for each of the N ﬁlters.
Consequently the fraction of separable dichotomies is f ( P, N ) .
5.1. L OCAL N ONLINEAR P OOLING
Often, nonlinear pooling operations are applied to downsample feature maps.
For concreteness, we will focus on one-dimensional signals in this section and relegate the proofs for two-dimensional signals (images) to Appendix A.3.2.
Let r ( x ) ∈ R N × D represent a feature map with N ﬁlters and length- D signals.
Consider a pooling operation P ( · ) which maps the D pixels in each feature map into new vectors of size D/k for some integer k .
Note that the pooled code is equivariant to the subgroup H =
Z D/k , in the sense that P ( π ( h ) r )
=
ρ ( h ) P ( r ) for any h ∈ H .
The representation ρ is the regular representation of the subgroup H .
We thus decompose G into cosets of size D/k : g = jh , where j ∈ Z k and h ∈
Z D/k .
The condition that a vector w separates the dataset is
[Equation]
We see that there are effectively k points belonging to each of the P orbits in the pooled code.
Since P ( · ) is nonlinear, the averaging trick utilized in Lemma 1 is no longer available.
However, we can obtain a lower bound on the capacity, f ( P, (cid:98) N/k
(cid:99) ) , from a simple extension of Cover’s original proof technique as we show in Appendix A.3.3.
Alternatively, an upper bound f ≤ f ( P, N ) persists since a w which satisﬁes Equation 4 must separate (cid:104) ρ ( h ) (cid:105) h ∈ H P ( r µ ) . This upper bound is tight when all k points (cid:104) ρ ( h ) (cid:105) h ∈ H P ( π ( j ) r µ ) coincide for each µ , giving capacity f ( P, N ) .
This is what occurs in the average pooling case where the upper bound f ≤ f ( P, N ) is tight.
Further, if we are only interested in the H -invariant capacity problem, the fraction of separable dichotomies is f ( P, N ) , since ρ is a regular representation of H as we show in 4.
5.1. G LOBAL AVERAGE POOLING
Lemma 1 shows that linear separability of π -manifolds is equivalent to linear separability after global average pooling (i.e. averaging the representation over the group).
This is relevant to CNNs, which typically perform global average pooling following the ﬁnal convolutional layer.
This strategy is efﬁcient: Lemma 1 implies that it allows the learning of optimal linear readout weights following the pooling operation, given only a single point from each π -manifold (i.e. no need for data augmentation).
Global average pooling reduces the problem of linearly classifying | G | P points in a space of dimension N to that of linearly classifying P points in a space of dimension dim V 0 .
4 α = P/N0 .0 .0 .0 [-.0 .
-].
[Figure] 0 .1 .c
a p a c i t y 0 .1 .1 .2 .2 .[-
-]
5 α = P/N0 .0 .0 .0 .0 .1 .f
[Figure]
) layer 0 .1 .1 .2 .2 .3 .
[Figure]
[Figure] [Figure]
[Figure]
pooling reduces capacity as predicted.
(c) f ( α ) after a random convolutional layer equivariant to the direct sum representation of Z 10 ⊕ Z 8 as deﬁned in Section 4.3, with P =and N 0 = 2 (# output channels).
5. I NDUCED REPRESENTATIONS
Induced representations are a fundamental ingredient in the construction of general equivariant neural network architectures (Cohen et al., 2019b).
Here we state our result, and relegate a formal deﬁnition of induced representations and the proof of the result to Appendix A.3.5.
Proposition 1.
Let π be a representation of a ﬁnite group induced from ρ .
Then the fraction of separable dichotomies of π -manifolds is equal to that of the ρ -manifolds.
D ISCUSSION AND C ONCLUSION
Equivariance has emerged as a powerful framework to build and understand representations that reﬂect the structure of the world in useful ways.
In this work we take the natural step of quantifying the expressivity of these representations through the well-established formalism of perceptron capacity.
We ﬁnd that the number of “degrees of freedom” available for solving the classiﬁcation task is the dimension of the space that is ﬁxed by the group action.
This has the immediate implication that capacity scales with the number of output channels in standard CNN layers, a fact we illustrate in simulations.
However, our results are also very general, extending to virtually any equivariant representation of practical interest – in particular, they are immediately applicable to GCNNs built around more general equivariance relations, as we illustrate with an example of a GCNN equivariant to direct sum representations.
We also calculate the capacity of induced representations, a standard tool in building GCNNs, and show how local and global pooling operations inﬂuence capacity.
The measures of expressivity explored here could prove valuable for ensuring that models with general equivariance relations have high enough capacity to support the tasks being learned, either by using more ﬁlters or by using representations with intrinsically higher capacity.
We leave this to future work.
While the concept of perceptron capacity has played an essential role in the development of machine learning systems (Sch¨olkopf & Smola, 2002;
Cohen et al., 2020) and the understanding of biological circuit computations (Brunel et al., 2004; Chapeton et al., 2012;
Rigotti et al., 2013; Brunel, 2016; Rubin et al., 2017; Pehlevan & Sengupta, 2017; Lanore et al., 2021;
Froudarakis et al., 2020), more work is wanting in linking it to other computational attributes of interest such as generalization.
A comprehensive picture of the computational attributes of equivariant or otherwise structured representations of artiﬁcial and biological learning systems will likely combine multiple measures, including perceptron capacity.
There is much opportunity, and indeed already signiﬁcant recent work (e.g. Sokolic et al.
R EPRODUCIBILITY S TATEMENT
Code for the simulations is attached as supplementary material zip ﬁle.
This code includes an environment.yml ﬁle that can be used to create a python environment identical to the one used by the authors.
The code generates all of the plots in the paper.
While the main text is self-contained with the essential proofs, proofs of additional results and further discussion can be found in the Appendices below.
These include
Appendix A.1 a derivation of the irreps for the regular representation of the cyclic group Z m .
Appendix A.2 a description of the irreps for the group
SO (3) and the resulting capacity.
Appendix A.3 a description of the construction of the GCNNs used in the paper, the methods for empirically measuring the fraction of linearly separable dichotomies, a description of local pooling, and complete formal proofs of the fraction of linearly separable dichotomies for these network representations (including with local pooling).
This appendix also includes more description of the induced representation and a formal proof of the fraction of linearly separable dichotomies.
Appendix A.3.1 also contains an additional ﬁgure, Figure A.1.
A A PPENDIX A. I RREPS FOR THE CYCLIC GROUP
Here we compute the irreps of representations π : Z m → GL( V ) of the cyclic group Z m over a real vector space V (see (Serre, 2014) for a derivation of the irreps when V is a complex vector space).
To ﬁnd the irreps, one can use the form for the eigenvalues and eigenvectors for circulant matrices, since all the π ( g ) are circulant.
This results in the simultaneous diagonalization π ( g ) =
V (1 ⊕ R (2 πg/m ) ⊕ R (4 πg/m ) ⊕ · · · ⊕ R (( m − 1) πg/m ))
V (cid:62) where V is the realvalued version of the discrete Fourier transform matrix (the columns are proportional to cosines and sines of varying frequencies, along with a column proportional to 1 m ).
Note that the 2 x 2 rotation matrices R (2 πgk/m ) are irreps for k (cid:54) = m/ 2 and k (cid:54) = 0 , since there is no one-dimensional subspace of R 2 that is invariant to R (2 πgk/m ) for all g .
The exception for k = m/ 2 if m is even, gives R (2 πgk/m ) =
( − 1)
g I , which corresponds to rotation ofdegrees.
The subspace span { (1 , 0) } is invariant to this action, so that R (2 πgk/m ) is not an irrep.
This representation can thus be reduced to an action on a one-dimensional subspace, represented by ( − 1) g .
The case k = 0 gives the trivial representation.
[Equation]
The special orthogonal group SO (3) on 3 dimensions (rotation group), the 3 × 3 orthogonal matrices with determinant +1 , can also be analyzed within our theory.
G-convolutional neural networks that are equivariant to SO (3) rotations have become of high interest in the physical sciences and computer vision where the objects of interest often respect these symmetries (Anderson et al., 2019;
Cohen et al., 2018; Esteves et al., 2018; Kondor et al., 2018a)
The irreducible representations have the form B k m where B k m are (2 k m + 1) ×
(2 k m + 1) block matrices, known as Wigner Dmatrices (Wigner, 1931).
The trivial irreps correspond to the one-dimensional irreps with k = 0 .
Thus, the SO (3) -invariant classiﬁcation capacity merely counts the number of trivial irreps which have N 0 = (cid:80) m δ k m , 0 .
The capacity is again f ( P, N 0 ) .
A. G- EQUIVARIANT CONVOLUTIONAL LAYERS A.3. S TANDARD CONVOLUTIONAL LAYERS
A convolutional layer consists of a set of N k × k (cid:48) ﬁlters F i that are convolved (technically crosscorrelated) with a stack of M W × L input tensors.
Here M is the number of input channels and
N the number of output channels.
The convolution runs each ﬁlter (i.e. takes the dot product at all possible positions) over each of the W × L input tensors, and the result is averaged across the
M input channels to produce the output of one output channel.
In the positions where the ﬁlters approach the edges of the input tensor, different choices can be made about how to handle these edge conditions.
The standard choice is to pad the edges with some number of zeros depending on the desired shape of the output tensor and run the convolution out to the end of the padded image.
Another possible choice is to loop the edges of the input tensor together, so that the ﬁlter is applied to the other side of the input tensor as it runs off the edge.
This periodic boundary condition allows us to write the convolution formally in terms of group actions, and to apply our theory directly.
When convolutions are not periodic, the resulting capacity increases somewhat but still follows the P/Nscaling of the periodic convolutions (Figure A.1).
For the random convolutional layers of Figure 2a, the input tensors are size 10 × 10 and the number of input channels are M = 3 , as for standard color images.
Each entry of these tensors is normally distributed with mean 0 .
The ﬁlters are also of size 10 × 10 with periodic boundary conditions, and are initialized according to a normal Xavier distribution with parameters that are the default for
Pytorch 1.9.
The convolution is implemented via the Pytorch 1.9 implementation of Conv2d with padding mode=“circular” and padding=0 in the case of periodic boundary conditions.
The bias term of the convolution is set to zero and the convolution is followed by a ReLU nonlinearity (blue line).
The resulting ﬁgures do not change appreciably for different choices of input tensor size, number of input channels, or size of ﬁlters (though note that the nonlinearity is essential for satisfying the general position condition of Theorem 1; otherwise, the capacity would be determined by the number
.2 .α
= P/N0 .0 .0 .0 .0 .1 .c
a p a c i t y
[Figure]
of input channels rather than number of output channels).
The output of this convolution is then fed through a 2 × 2 max pooling layer (orange line in Figure 2a), provided by Pytorch 1.9’s MaxPool2d.
The pretrained VGG-11 layers used in Figure 2b and Figure A.1 are taken from liukuang (2017).
The ﬁrst convolutional block (blue line) consists of 3 × 3 pretrained ﬁlters applied to CIFARimage tensors randomly selected from the validation set and normalized in the same way they are normalized during training (see liukuang (2017) for details).
These images are of size 32 × 32 and have M = 3 input channels, followed by a batch normalization layer in evaluation mode (ﬁxed parameters), and then followed by a ReLU nonlinearity.
The boundary conditions of these convolutions are set to be periodic in Figure 2b and nonperiodic with a zero padding sizes of 1 in Figure A.1, and the bias term is set to zero.
The batch normalization is an element-wise operation and so equivariant to the representations we consider – thus this operation is not expected and is not observed to affect the perceptron capacity.
This convolutional block is then followed by a 2 × 2 max pooling layer (orange line).
Finally, another
The fraction of linearly separable dichotomies is measured empirically by using the scikit-learn
LogisticRegression implementation of logistic regression, with a tolerance value of tol=1e-18 and an inverse regularization value of C=1e8.
The maximum number of iterations is set to 500.
An intercept (i.e. bias) is not used for this ﬁt.
To formally prove that the fraction of separable dichotomies is f ( P, N ) for standard periodic convolutional layers, ﬁrst note that the convolution is equivariant with respect to cyclic permutation of the inputs and of the outputs.
The representation for cyclically permutating the output tensor can be written (cid:76) Nk
=1 π where π is the representation that cyclically permutes the entries of W × L matrices.
Since each copy of π contains one trivial irrep in its decomposition into a direct sum of irreps, the direct sum (cid:76) Nk
=1 π contains N trivial irreps in its decomposition.
The ﬁnal step to use Theorem 1 is to argue that the centroids of the manifolds are in general position.
Since a nonlinearity (ReLU) is applied to the output of the convolution, and since there is no particular structure in the convolutional ﬁlters beyond possibly sparsity, we can generically expect this to be the case.
A.3. L OCAL P OOLING
First we prove an extension of Lemma 1 to equivariant linear maps.
This will be used to show that average pooling does not affect the capacity of the regular representation of Z m .
Lemma 2.
Let π be a representation of the group G and suppose the matrix P is equivariant with respect to the restriction of π to a subgroup H ⊆ G , so that for all h ∈ H P π ( h )
=
ρ ( h ) P for some representation ρ of H .
Let R denote a set of representatives of G/H .
Then we have the following equivalence.
[Equation]
Proof.
For the forward implication [Equation] we write the coset decomposition g = hg (cid:48) of g and average over H to ﬁnd
[Equation]
For the backward implication
[Equation]
suppose y µ w (cid:62) P (cid:104)
π
( h ) (cid:105)
h ∈ H π ( g (cid:48) )
r µ > 0 for all representatives g (cid:48) ∈
R , and deﬁne ˜
w = (cid:104) ρ ( h ) (cid:105) (cid:62) h ∈
H w .
For any g ∈ G , take a coset decomposition g = hg (cid:48) for h ∈ H and g (cid:48) ∈ R .
We then have
[Equation]
The implication follows.
Lemma 3.
For the regular representation of G = Z D , a local average pooling over windows of size k generates a matrix P which is equivariant with respect to the subgroup H =
Z D/k with the property that
[Equation]
where a > 0 is a positive constant.
Proof.
First, note that the new pooled code is the regular representation of H since shifts of size mk in the original feature map corresponds to shifts of length m in the pooled code.
Thus P is equivariant to H =
Z D/k .
Next we note the following two facts
[Equation]
a (cid:48) and a (cid:48)(cid:48) are positive constants and 1 D and 1 D/k are the D and D/k dimensional vector of all ones, respectively.
Thus, we have that P (cid:104) π ( h ) (cid:105) h ∈ H = a P (cid:104) π ( g ) (cid:105) g ∈ G where a is a positive constant.
Theorem 2.
The fraction of linearly separable dichotomies of a CNN pooling layer with N ﬁlters after average pooling from feature maps with the size of the input image W × L to pooled feature maps of size W/k × L/k is f ( P, N ) , i.e. no capacity is lost due to local average pooling.
Proof.
The CNN layer before pooling is a regular representation π of the full group G =
Z W × Z L , applied to each of the M input channels via a direct a sum (cid:76) Mj =1 π .
The layer after pooling is a regular representation ρ of the subgroup H =
Z W/k × Z L/k , also applied to the output channels via a direct sum (cid:76) Nj =1 ρ .
Let R be a set of representatives of G/H .
Since P is equivariant to π and ρ over H we have by the previous two lemmas that
[Equation]
Thus the capacity is determined by the rank of (cid:104) ρ ( h ) (cid:105) h ∈ H , assuming the (cid:104) ρ ( h ) (cid:105) h ∈ H P r µ are in general position.
Since each ρ is a copy of the regular representation for H , the rank of (cid:104) (cid:76) Nj =1 ρ ( h ) (cid:105) h ∈ H is merely N .
Thus the fraction of linearly separable dichotomies is f ( P, N ) , the same as the capacity before pooling.
Now we prove local pooling operations in a standard CNN preserve a regular representation of a subgroup of the cyclic group.
Lemma 4.
Suppose P is a local pooling operation on two-dimensional signals (CNN feature maps), and that π is the regular representation of a group G =
Z W × Z L on code a ( x ) .
A pooled feature map r = P ( a ) which acts on k × k windows of a is a regular representation of the subgroup
H =
Z W/k × Z L/k .
Proof.
Suppose an equivariant feature map a ( x ) ∈ R W × L × N has corresponding regular representation of the group G
= Z W × Z L for each of the N ﬁlters.
Consider any local pooling operation
P ( · ) (such as average or maximum) which acts on k × k patches where k divides both W and L .
[Equation]
P [Equation] Note that for k > 1 , r ( x ) is no longer equivariant to G since the representation does not satisfy the homomorphism property for shifts with length (cid:96) not divisible by k .
However, the new code is equivariant to a subgroup H =
Z W/k × Z L/k , namely vertical and horizontal shifts with length divisible by k .
Let π x nk,mk represent a vertical shift of the image x by nk pixels and horizontal shift by mk pixels.
Note that a ij,h ( π x nk,mk x ) =
a i + nk,j + mk ( x ) since a ( x ) is equivariant.
Then, the h -th pooled feature map transforms as
[Equation]
We thus ﬁnd that the code is a regular representation of the subgroup of the cyclic translations
H =
{ ( nk, mk ) } n ∈ [ H/k ] ,m ∈
[ W/k ] .
This new group G (cid:48) has dimension | H | = 1 k 2 | G | .
A.3. L OWER B OUND AND U PPER B OUND ON C APACITY FOR N ONLINEAR P OOLING
Theorem 3.
Suppose a code which is equivariant to a ﬁnite group G is pooled to a new code which is equivariant to a ﬁnite subgroup H ⊆ G .
Suppose the number of trivial dimensions in the original G -equivariant code is N 0 .
Then the fraction of linearly separable dichotomies on the G -invariant problem for the pooled code is at least f ( P, (cid:98) N 0 /k (cid:99) ) where k = | G/H | .
Similarly the fraction is at most f ( P, N 0 ) .
Proof.
The pooled code, by assumption, has the property P ( π ( hg (cid:48) ) r )
=
ρ ( h ) P ( π ( g (cid:48) ) r ) for any h ∈ H and g (cid:48) ∈ R , where R is a set of representatives of G/H .
The G -invariant separability condition amounts to the proposition
[Equation]
in other words, a solution on the right hand side affords a solution over all of the manifolds generated in the input space.
We see that, this requires considering if this particular dichotomy is linearly separble on the Pk anchor points (cid:104) ρ ( h ) (cid:105)
h ∈ H P ( π ( g (cid:48) )
r µ ) .
The simplest strategy to obtain an upper bound is to consider what happens when a single new manifold is added.
We see that when a single new base point r is added it corresponds to k new points in the orbit (cid:104) ρ (cid:105) P ( π ( g (cid:48) ) r ) for all g (cid:48) ∈ R .
Suppose that (cid:104) ρ ( h ) (cid:105) h ∈ H has rank N 0 .
Let C ( P, N 0 ) represent the number of linearly separable dichotomies for P G -orbits in N 0 trivial dimensions.
Upon the addition of the k new points ( P → P + 1 ), we ﬁnd that some of the pre-existing separable dichotomies give a new separable dichotomy.
This can be guaranteed to occur when a w separates the old dichotomy and has w (cid:62) (cid:104) ρ (cid:105) ·
P ( π ( g (cid:48) ) r ) = 0 for the new anchor point r (but this condition is not necessary for a new dichotomy to be separable).
This condition means that the original dichotomy is separable in the N 0 − k dimensional subspace { w : w · P ( π ( g (cid:48) ) r ) = 0 } .
By making inﬁnitesimal adjustment to this w the correct label on this new orbit can be achieved without altering the labels on any other dichotomy.
Since this argument gives a sufﬁcient but not necessary condition to generate a new dichotomy, we obtain the following inequality
[Equation]
Solving this recursion gives the capacity f Pooled ( P, N 0 ) ≥ f Cover ( P, (cid:98) N 0 /k (cid:99) ) .
The greatest capacity occurs in the special case where (cid:104) ρ (cid:105) P
( π ( g (cid:48) ) r )
= (cid:104) ρ (cid:105) P ( r ) .
In this case, the usual counting theorem applies giving a fraction of separable dichotomies of f ( P, N 0 ) .
This is achieved, for instance in average pooling as we showed in Theorem 2.
A.3. D IRECT SUM EQUIVARIANT CONVOLUTIONAL LAYERS
Here we describe how to build a convolutional layer architecture that is equivariant with respect to the regular representation in the input space and the direct sum representations introduced in
Section 4.3 in the output space.
For the following we assume that m 1 and m 2 are coprime, though see the footnote in Section 4.3 for a discussion of how to loosen this requirement.
The input data is a W × L × M tensor where M is the number of input channels.
The ﬁrst step is to simply take the output of a standard convolution (in our simulations we also apply a ReLU nonlinearity) applied to this input with periodic boundary conditions, resulting in a W × L ×
N tensor where N is the number of output channels.
The next step is to, for each of the output channels, take an average (or maximum) between entries spaced m 1 entries apart horizontally or vertically in the matrix, resulting in an m 1 × m 1 matrix.
In our simulations we took averages rather than maximums.
This is repeated for the other number m 2 , resulting in an m 2 × m 2 matrix.
This is repeated for every output channel, resulting in N matrices of size m 1 × m 1 and N matrices of size m 2 × m 2 .
Finally, the resulting matrices are ﬂattened and appended into an ( m 21 + m 22 ) × N matrix, and the result is passed through a nonlinearity (ReLU).
As the input tensor is cyclically permuted according to a regular representation π of Z m 1 m 2 , the output of this equivariant convolutional layer permutes according to the representation π (1) ⊕ π (2) where π (1) is the regular representation of Z m 1 and π (2) is the regular representation of Z m 2 .
The proof that the fraction of separable dichotomies is given by f ( P, 2 N ) follows the same proof as for the standard periodic convolutions in Appendix A.3.1.
Instead of a direct sum (cid:76) Nk
=1 π
we get a direct sum (cid:76)
Nk =
( π (1) ⊕ π (2) ) .
Each of the π (1) ⊕ π (2) contain two trivial irreps in its decomposition, so that the ﬁnal fraction is f ( P, 2 N ) .
A.3. I NDUCED REPRESENTATIONS
First we state the deﬁnition of an induced representation.
Let H be a subgroup of a ﬁnite group
G and let ρ : H → GL( W ) be a representation of H .
Let V be the linear space of functions f : G → V such that f ( gh )
=
ρ ( h ) f
( g ) for all h ∈ H and g ∈ G .
We now deﬁne the induced representation π : G → GL( V ) to be the representation which satisﬁes π ( g (cid:48) )
f ( g ) =
f ( gg (cid:48) ) .
For intuition, note that every element of g can be written g
= rh where r is a representative for a coset in G/H and h ∈ H .
This is because the cosets G/H partition G and the action of H stays within a coset; hence r selects out the coset, and h goes to the desired element of the coset: g = rh .
With this decomposition, the action of π is then π ( rh ) f
( g ) =
ρ ( h ) f
( gr ) .
Hence the r component under π has the effect of permuting to the new coset that gr belongs in, and the h component under π then has the effect ρ ( h ) on the resulting vector f ( gr ) that we originally speciﬁed.
This is the most natural way to get a representation of G from a representation of H .
In the case of ﬁnite groups, one can think of the r component as permuting a set of isomorphic copies of V , each copy corresponding to a different coset.
To compute the capacity of induced representations, and so prove Proposition 1, we use Frobenius reciprocity of characters.
Recall that the character θ of a representation π : G → GL( V ) is the map θ : G → C induced by the trace: θ ( g ) =
Tr( π ( g )) .
Now let θ be the character of ρ : H → GL( V ) and let θ G be the character of the induced representation.
Then (cid:104)
θ G ( g ) (cid:105) g ∈ G = (cid:104)
θ ( h ) (cid:105) h ∈ H by Frobenius reciprocity of characters (Mackey, 1970).
The average of the character is the number of trivial representations contained in the decomposition of the representation (see Serre (2014)).
Hence the capacity of the induced representation is equal to the capacity of ρ .
The existence of extensions beyond ﬁnite groups is not clear to the authors, but we welcome information if such exists.