I S DEEPER BETTER ? I T DEPENDS ON LOCALITY OF RELEVANT FEATURES
Anonymous authors
Paper under double-blind review
A BSTRACT
It has been recognized that a heavily overparameterized artiﬁcial neural network exhibits surprisingly good generalization performance in various machinelearning tasks.
Recent theoretical studies have made attempts to unveil the mystery of the overparameterization.
In most of those previous works, the overparameterization is achieved by increasing the width of the network, while the effect of increasing the depth has remained less well understood.
In this work, we investigate the effect of increasing the depth within an overparameterized regime.
To gain an insight into the advantage of depth, we introduce local and global labels as abstract but simple classiﬁcation rules.
It turns out that the locality of the relevant feature for a given classiﬁcation rule plays a key role; our experimental results suggest that deeper is better for local labels, whereas shallower is better for global labels.
We also compare the results of ﬁnite networks with those of the neural tangent kernel (NTK), which is equivalent to an inﬁnitely wide network with a proper initialization and an inﬁnitesimal learning rate.
It is shown that the NTK does not correctly capture the depth dependence of the generalization performance, which indicates the importance of the feature learning, rather than the lazy learning.
I NTRODUCTION
Deep learning has achieved unparalleled success in various tasks of artiﬁcial intelligence such as image classiﬁcation (Krizhevsky et al., 2012; LeCun et al., 2015) and speech recognition (Hinton et al., 2012).
Remarkably, in modern machine learning applications, impressive generalization performance has been observed in an overparameterized regime, in which the number of parameters in the network is much larger than that of training data samples.
Contrary to what we learn in classical learning theory, an overparameterized network ﬁts random labels and yet generalizes very well without serious overﬁtting (Zhang et al., 2017).
We do not have general theory that explains why deep learning works so well.
Recently, the learning dynamics and the generalization power of heavily overparameterized wide neural networks have extensively been studied.
It has been reported that training of an overparameterized network easily achieves zero training error without getting stuck in local minima of the loss landscape (Zhang et al., 2017; Baity-Jesi et al., 2018).
Mathematically rigorous results have also been obtained (Allen-Zhu et al., 2019; Du et al., 2019).
From a different point of view, theory of the neural tangent kernel (NTK) has been developed as a new tool to investigate an overparameterized network with an inﬁnite width (Jacot et al., 2018; Arora et al., 2019), which simply explains the reason why a sufﬁciently wide neural network can achieve a global minimum of the training loss.
As for generalization, a “double-descent” phenomenon has attracted much attention (Spigler et al., 2019; Belkin et al., 2019).
The standard bias-variance tradeoff picture predicts a U-shaped curve of the test error (Geman et al., 1992), but instead we ﬁnd the double-descent curve, which implies that an increased model capacity beyond the interpolation threshold results in improved performance.
This ﬁnding triggered detailed studies on the behavior of the bias and variance in an overparameterized regime (Neal et al., 2019; D’Ascoli et al., 2020).
The double-descent phenomenon is not explained by traditional complexity measures such as the Vapnik-Chervonenkis dimension and the Rademacher complexity (Mohri et al., 2018), and hence one seeks for new complexity measures of deep neural networks that can prove better generalization bounds (Dziugaite & Roy, 2017;
Neyshabur et al., 2017; 2019; Arora et al., 2018; Nagarajan & Kolter, 2017; P´erez et al., 2019).
These theoretical efforts mainly focus on the effect of increasing the network width, but beneﬁts of the network depth remain unclear.
It is known that expressivity of a deep neural network grows exponentially with the depth rather than the width (Poole et al., 2016).
See also Bianchini & Scarselli (2014); Mont´ufar et al.
However, it is far from clear whether exponential expressivity does lead to better generalization (Ba & Caruana, 2014; Becker et al., 2020).
It is also nontrivial whether typical problems encountered in practice require such high expressivity.
Although some works (Eldan & Shamir, 2016; Safran & Shamir, 2017) have shown that there exist simple and natural functions that are efﬁciently approximated by a network with two hidden layers but not by a network with one hidden layer, a recent work (Malach & Shalev-Shwartz, 2019) has demonstrated that a deep network can only learn functions that are well approximated by a shallow network by using a gradient-based optimization algorithm, which indicates that beneﬁts of the depth are not due to high expressivity of deep networks.
Some other recent works have reported no clear advantage of the depth in an overparameterized regime (Geiger et al., 2019a;b).
To gain an insight into the advantage of the depth, in the present paper, we report our experimental study on the depth and width dependences of generalization in abstract but simple, well-controlled classiﬁcation tasks with fully connected neural networks.
We introduce local labels and global labels , both of which give simple mappings between inputs and output class labels.
By “local”, we mean that the label is determined only by a few components of the input vector.
On the other hand, a global label is determined by a global feature that is expressed as a sum of local quantities, and hence all components of an input contribute to the global label.
Our experiments show strong depth-dependences of the generalization error for those simple input-output mappings.
In particular, we ﬁnd that deeper is better for local labels, while shallower is better for global labels .
This result implies that the depth is not always advantageous, but the locality of relevant features would give us a clue for understanding the advantage the depth brings about.
We also compare the generalization performance of a trained network of a ﬁnite width with that of the kernel method with the NTK.
The latter corresponds to the inﬁnite-width limit of a fully connected network with a proper initialization and an inﬁnitesimal learning rate (Jacot et al., 2018), which is referred to as the NTK limit.
In the NTK limit, the network parameters stay close to their initial values during training, which is called the lazy learning (Chizat et al., 2019), and it is known that a wide but ﬁnite network can still be in the lazy learning regime for sufﬁciently small learning rates (Ji & Telgarsky, 2019; Chen et al., 2019).
We however ﬁnd that even if the width increases, in some cases the generalization error at an optimal learning rate does not converge to the NTK limit.
In such a case, a ﬁnite-width network shows much better generalization compared with the kernel learning with the NTK.
This ﬁnding indicates the importance of the feature learning , in which network parameters change to learn relevant features.
S ETTING
We consider a classiﬁcation task with a training dataset D = { ( x ( µ ) , y ( µ ) ) : µ = 1 , 2 , . . .
, N } , where x ( µ ) ∈ R
d is an input data and y ( µ ) ∈ { 1 , 2 , . . .
, K } is its label.
In this work, we consider the binary classiﬁcation, K = 2 , unless otherwise stated.
2. D ATASET
Each input x = ( x 1 , x 2 , . . .
, x d ) T is a d -dimensional vector taken from i.i.d.
Gaussian random variables of zero mean and unit variance, where a T is the transpose of a vector a .
For each input x , we assign a label y
according to one of the following rules.
k - LOCAL LABEL
We randomly ﬁx integers { i 1 , i 2 , . . .
i k } with 1 ≤ i 1 < i 2 < · · · < i k ≤ d .
In the “ k -local” label, the relevant feature is given by the product of the k components of an input x , that is, the label y is determined by
[Equation]
This label is said to be local in the sense that y is completely determined by just the k components of an input x . 1 For fully connected networks considered in this paper, without loss of generality, we can choose i 1 = 1 , i 2 = 2 ,...
i k
= k
because of the permutation symmetry with respect to indices of input vectors.
k - GLOBAL LABEL
We again ﬁx 1 ≤ i 1 < i 2 < · · · < i k ≤ d .
Let us deﬁne
M = d
X j = x j
+ i 1 x j
+
[Equation]
x j + i k , (2) where the convention x d + i
=
x i is used.
The k -global label y for x is deﬁned by
[Equation]
if M
[Equation]
otherwise
[Equation]
The relevant feature M for this label is given by a uniform sum of the product of k components of the input vector.
Every component of x contributes to this “ k -global” label, in contrast to the k -local label with k < d .
2. N ETWORK ARCHITECTURE
In the present work, we consider fully connected feedforward neural networks with L hidden layers of width H .
We call L and H the depth and the width of the network, respectively.
The output of the network f ( x ) for an input vector x ∈ R d is determined as follows:
[Equation]
where ϕ ( x ) = max { x, 0 } is the component-wise ReLU activation function, z ( l ) is the output of the l th layer, and
[Equation]
are the weights and the biases, respectively.
Let us denote by w the set of all the weights and biases in the network.
We focus on an overparameterized regime, i.e., the number of network parameters (the number of components of w ) exceeds N , the number of training data points.
2. S UPERVISED LEARNING
The network parameters w are adjusted to correctly classify the training data.
It is done by minimizing the softmax cross-entropy loss L ( w ) given by
[Equation]
where the i th component of f ( x ) is denoted by f i ( x ) .
The main results of our paper do not change for other standard loss functions such as the mean-squared error.
The training of the network is done by the stochastic gradient descent (SGD) with learning rate η and the mini-batch size B .
That is, for each mini-batch B ⊂ D with |B| =
B , the network parameter w t at time t is updated as
[Equation]
Throughout the paper, we ﬁx B = 50 .
Meanwhile, we optimize η > 0 before training (explain the detail later).
Biases are initialized to be zero, and weights are initialized using the Glorot initialization (Glorot & Bengio, 2010).
The trained network classiﬁes an input x µ to the class ˆ y ( µ ) =
arg max i ∈{ 1 , 2 ,...,K } f i ( x ( µ ) ) .
Let us then deﬁne the training error as
[Equation]
that is the miss-classiﬁcation rate for the training data D .
We train our network until E train = 0 is achieved, i.e., all the training data samples are correctly classiﬁed, which is possible in an overparameterized regime.
For a training dataset D , we ﬁrst perform the 10-fold cross validation to optimize the learning rate η under the Bayesian optimization method (Snoek et al., 2012), and then perform the training via the SGD by using the full training dataset.
In the optimization of η , we try to minimize the missclassiﬁcation ratio for the validation data.
The generalization performance of a trained network is measured by computing the test error.
We prepare the test data D test = { ( x ′ ( µ ) , y ′ ( µ ) ) : µ = 1 , 2 , . . .
, N test } independently from the training data D .
The test error E test is deﬁned as the miss-classiﬁcation ratio for D test , i,.e.,
[Equation]
where ˆ y ′ ( µ ) =
arg max i f i ( x ′ ( µ ) ) is the prediction of our trained network.
In our experiment discussed in Sec. 3, we ﬁx N test = 10 5 .
2. N EURAL T ANGENT K ERNEL
Following Arora et al. and Cao & Gu (2019), let us consider a network of depth L and width H whose biases { b ( l ) } and weights { w ( l ) } are randomly initialized as b ( l ) i = β ˜ b ( l ) i with ˜ b ( l ) i ∼ N (0 , 1) and w ( l ) ij = p 2 /n l − 1 ˜ w ( l ) ij with ˜ w ( l ) ij ∼ N (0 , 1) for every l respectively, where n l is the number of neurons in the l th layer, i.e., n 0 = d , n 1 = n 2 = · · · = n L = H .
Let us denote by ˜ w the set of all the scaled weights { ˜ w ( l ) } and biases { ˜ b ( l ) } .
The network output is written as f ( x, ˜ w ) . When the network is sufﬁciently wide and the learning rate is sufﬁciently small, the network parameters ˜ w stay close to their randomly initialized values ˜ w 0 during training, and hence f ( x, ˜ w ) is approximated by a linear function of ˜ w − ˜ w 0 : f ( x, ˜ w ) = f ( x, ˜ w 0 ) + ∇ ˜ w f ( x, ˜ w ) | ˜ w
=
˜
w 0 · ( ˜ w − ˜ w 0 ) .
As a result, the minimization of the mean-squared error L MSE = (1 /N )
P
{+Nµ =
+}
[ f (
[Equation]
where ~y ( µ ) ∈ R K is the one-hot representation of the label y ( µ ) , is equivalent to the kernel regression with the NTK Θ ( L ) ij ( x, x ′ )
( i, j = 1 , 2 , . . .
, K ) that is deﬁned as
[Equation]
where E ˜ w denotes the average over random initializations of ˜ w (Jacot et al., 2018).
The parameter β controls the impact of bias terms, and we set β = 0 .
1 in our numerical experiment following Jacot et al.
By using the ReLU activation function, we can give an explicit expression of the NTK that is suited for numerical calculations.
Such formulas are given in Appendix A.
[Figure]
It is shown that the NTK takes the form Θ ( L ) ij ( x, x ′ )
=
δ i,j Θ ( L ) ( x, x ′ ) , and the minimization of the mean-squared error with an inﬁnitesimal weight decay yields the output function
[Equation]
(11) where K − 1 is the inverse matrix of the Gram matrix K µν =
Θ ( L ) ( x ( µ ) , x ( ν ) ) .
An input data x is classiﬁed to ˆ y = arg max i ∈{ 1 , 2 ,...,K }
f NTK i ( x ) .
E XPERIMENTAL RESULTS
We now present our experimental results.
For each data point, the training dataset D is ﬁxed and we optimize the learning rate η via the 10-fold cross validation with the Bayesian optimization method (we used the package provided in Nogueira (2014)).
We used the optimized η to train our network.
At every 50 epochs we compute the training error E train , and we stop the training if E train = 0 .
For the ﬁxed dataset D and the optimized learning rate η , the training is performed 10 times and calculate the average and the standard deviation of test errors E test .
3. 1- LOCAL AND 1- GLOBAL LABELS
In the 1-local and 1-global labels, the relevant feature is a linear function of the input vector.
Therefore, in principle, even a linear network can correctly classify the data.
Figure 1 shows the generalization errors in nonlinear networks of the varying depth and width as well as those in the linear perceptron (the network of zero depth).
The input dimension is set to be d = 1000 .
We also plotted test errors calculated by the NTK, but we postpone the discussion about the NTK until Sec. 3.3.
Figure 1 shows that in both 1-local and 1-global labels, the test error decreases with the network width, and a shallower network ( L = 1 ) shows better generalization compared with a deeper one ( L = 7 ).
The linear perceptron shows the best generalization performance, which is natural because it is the simplest network that is capable of learning the relevant feature associated with the 1-local or 1-global label.
Remarkably, test errors of nonlinear networks ( L = 1 and L = 7 ) are not too large compared with those of the linear perceptron, although nonlinear networks are much more complex than the linear perceptron.
For a given network architecture, we do not see any important difference between the results for 1-local and 1-global labels, which would be explained by the fact that these labels are transformed to each other via the Fourier transformation of input vectors.
[Figure]
3. O PPOSITE DEPTH DEPENDENCES FOR k - LOCAL AND k - GLOBAL LABELS WITH k ≥
For k ≥ 2 , it turns out that experimental results show opposite depth dependences for k -local and k -global labels.
Let us ﬁrst consider k -local labels with k ≥ 2 .
Figure 2 (a) and (b) show test errors for varying N in various networks for the 2-local and the 3-local labels, respectively.
The input dimension d is set to be d = 500 in the 2-local label and d = 100 in the 3-local label.
We see that the test error strongly depends on the network depth.
A deeper network ( L = 7 ) generalizes better than a shallower one ( L = 1 ).
It should be noted that for d = 500 , the network of L =and H = 2000 contains about 10 6 trainable parameters, the number of which is much larger than that of trainable parameters ( ≃ 10 5 ) in the network of L = 7 and H = 100 .
In spite of this fact, the latter outperforms the former in the 2-local label as well as in the 3-local label with large N , which implies that increasing the number of trainable parameters do not necessarily implies better generalization.
In k -local labels with k ≥ 2 , the network depth is more strongly correlated to generalization compared with the network width.
From Fig.
2 (b), it is obvious that the network of L = 7 and H = 100 fails to learn the 3-local label for small N .
We also see that error bars of the test error are large in the network of L = 7 and
H = 100 .
The error bar represents the variance due to initialization and training.
By increasing the network width H , both variances and test errors decrease.
This result is consistent with the recent observation in the lazy regime that increasing the network width results in better generalization because it reduces the variance due to initialization (D’Ascoli et al., 2020).
Next, we consider k -global labels with k = 2 and 3.
The input dimension d is set as d = 100 for the 2-global label and d = 40 for the 3-global label.
We plot test errors against N in Fig.
2 for (c) the 2-global label and (d) the 3-global label.
Again we ﬁnd strong depth dependences, but now shallow networks ( L = 1 ) outperform deep ones ( L = 7 ), which is contrary to the results for k -local labels. For L = 7 , we also ﬁnd strong width dependences; the test error of a wider network more quickly
t e s t
e rr o r depth 2-local2-global decreases with N .
In particular, in the 3-global label, an improvement of the generalization with N is subtle for L = 7 and H = 100 .
By increasing the width, the decrease of the test error with N becomes much faster [see the result for L = 7 and H = 500 in Fig.
2 (d)].
To see more details of the effect of depth, we also plot the depth dependence of the test error for ﬁxed training data samples.
We prepare N = 10000 training data samples for the 2-local and 2global labels, respectively.
The input dimension is d = 500 for the 2-local label and d = 100 for the 2-global label.
By using the prepared training data samples, networks of the depth L and the width
H = 500 are trained up to L = 10 .
The test errors of trained networks are shown in Fig.
In the 2-local label, the test error decreases with L , whereas the test error increases with L in the 2-global label.
Thus, Fig. 3 clearly shows the opposite depth dependences for local and global labels.
3. C OMPARISON BETWEEN FINITE NETWORKS AND NTK S
In Figs.
1 and 2, test errors calculated by using the NTK are also plotted.
In the case of k =(Fig. 1) and the 2-global label [Fig. 2 (c)], the generalization performance of the NTK is comparable or lower than that of ﬁnite networks.
A crucial difference is seen in the case of k -local label with k = 2 and 3 and the 3-global label.
In
Fig.
2 (a) and (b), we see that the NTK almost completely fails to classify the data, although ﬁnite networks succeed in doing so.
In the case of the 3-global label, the NTK of depth L = 7 correctly classiﬁes the data, while the NTK of depth L = 1 fails [see Fig.
2 (d)].
In those cases, the test error calculated by a ﬁnite network does not seem to converge to that obtained by the NTK as the network width increases.
The NTK has been proposed as a theoretical tool to investigate the inﬁnite-width limit, but it should be kept in mind that the learning rate has to be sufﬁciently small to achieve the NTK limit (Jacot et al., 2018; Arora et al., 2019).
The discrepancy between a wide network and the NTK in Fig.
2 stems from the strong learning-rate dependence of the generalization performance.
In our experiment, the learning rate has been optimized by performing the 10-fold cross validation.
If the optimized learning rate is not small enough for each width, the trained network may not be described by the NTK even in the inﬁnite-width limit.
In Fig.
4 (a), we plot the learning-rate dependence of the test error for the 2-local label and the 2-global label in the network of the depth L = 1 and the width H = 2000 .
We observe a sharp learning-rate dependence in the case of the 2-local label in contrast to the case of the 2-global label.
In Fig.
4 (b), we compare the learning-rate dependences of the test error for L = 1 and L = 7 in the case of the 3-global label (in both cases H = 2000 ).
We see that the learning-rate dependence for L = 1 is much stronger than that for L = 7 , which is consistent with the fact that the NTK fails only for L = 1 .
It should be noted that Fig.
4 (b) shows that the deep network ( L = 7 ) outperforms the shallow one ( L = 1 ) in the regime of small learning rates, while the shallow one performs better than the deep one at their optimal learning rates.
[Figure] Figure 4 also shows that the test error for a sufﬁciently small learning rate approaches the one obtained by the corresponding NTK.
Therefore, the regime of small learning rates is identiﬁed as a lazy learning regime, while larger learning rates correspond to a feature learning regime.
An important message of Fig. 4 is that we should investigate the feature learning regime, rather than the lazy learning regime, to correctly understand the effect of depth on generalization.  D ISCUSSION ON THE OBSERVED DEPTH DEPENDENCE It is crucial for our problem to understand why deeper (shallower) is better for local (global) labels.
As a possible mechanism, we suggest that local features can naturally be detected with the help of the chaotic signal propagation through a deep network (Poole et al., 2016), i.e., multiplicative growth of a small perturbation to an input x across each layer.
An important distinction between the k -local label and the k -global label with k ≪ d lies in their stability under perturbations.
We can typically change the k -local label of an input x ∈ R d by moving x to ˜ x
= x + z with k z k
= O (1) , where z ∈ R d is called a perturbation.
On the other hand, a stronger perturbation k z k =
˜ O ( √ d ) is typically needed to change the k -global label.
A recent study by De Palma et al.
(2019) shows that a randomly initialized pre-trained wide neural network generates a label such that a perturbation z with k z k
= ˜ O
( √ d ) is typically required to change it.
It indicates that ˜ O ( √ d ) is a natural scale of resolution for a pre-trained random neural network.
Compared with it, learning the k -local label requires ﬁner resolution: two close inputs x and x ′ with k x − x ′ k
= O (1) should result in distinct outputs f ( x ) and f ( x ′ ) when x and x ′ have different labels.
A deep neural network can naturally implement such a situation via chaotic signal propagation, and hence depth is beneﬁcial to learning local features.
On the other hand, the k -global label is as stable as a typical label generated by a pre-trained network, and the chaoticity is not needed: two close inputs x and x ′ with k x − x ′ k ≪
˜ O ( √ d ) typically have the same label and therefore the outputs f ( x ) and f ( x ′ ) are also close to each other.
In this case, the chaoticity may bring about unnecessary high resolution and therefore be rather disadvantageous: a shallower network is better for the k -global label.
This is a potential explanation of the result obtained in
Sec. 3.2.
Although the chaotic signal propagation was originally discussed to explain high expressivity of deep networks (Poole et al., 2016), we emphasize that the beneﬁt of depth in local labels is not due
Hamming distance, but we expect that it also holds for continuous inputs with the Euclidean distance.
to high expressivity since learning the k -local label with k ≪ d does not require high expressivity.
Nevertheless, the chaoticity of deep neural networks may play a key role.
To support the above intuitive argument, we experimentally show in Appendix B that a deeper network has a tendency towards learning more local features even in training random labels .
That is, even for learning a completely structureless dataset, leaned features in a deep network tend to be more local compared with those in a shallow one, which is possibly due to chaoticity of deep networks and potentially explains the result of Sec. 3.2.
C ONCLUSION
In this work, we have studied the effect of increasing the depth in classiﬁcation tasks.
Instead of using real data, we have employed abstract setting with random inputs and simple classiﬁcation rules because such a simple setup helps us to understand under what situations deeper networks perform better or worse.
We ﬁnd that the locality of relevant features for a given classiﬁcation rule plays a key role.
In Sec. 4, we have proposed that the chaotic signal propagation in deep networks gives a possible mechanism to explain why deeper is better in local labels. It is also an interesting observation that shallower networks do better than deeper ones for the kglobal label.
This result shows that the depth is not always beneﬁcial. Although our dataset with the k -local or k -global label is highly artiﬁcial, there are some realistic examples in which k -global label is relevant, i.e., thermodynamic systems in physics .
Thermodynamic variables in physics are usually written in the form of Eq.
(2), i.e., a sum of local quantities over the entire volume of the system, and hence are regarded as global features. Our result therefore implies that a shallower network is more adequate for machine learning of thermodynamic systems.
In Appendix C, we demonstrate that it is indeed the case in a classiﬁcation problem of spin conﬁgurations of the Ising model, which is a simple model of ferromagnets Nishimori & Ortiz (2011).
In typical image classiﬁcation tasks, local features would be important and deeper networks do better, but we expect that there are some real datasets in which global features are more important like an example discussed in Appendix C. We believe that our ﬁnding is relevant to real dataset, apart from physics models discussed above, but it is an important future problem to understand how to connect our empirical results with machine learning of real dataset.
A E XPLICIT EXPRESSION OF THE NTK
We consider a network whose biases { b ( l ) } and weights { w ( l ) } are randomly initialized as b ( l ) i = β ˜ b ( l ) i with ˜ b
( l ) i ∼ N (0 , 1) and w ( l ) ij =
p 2 /n l − 1 ˜ w ( l ) ij with ˜ w ( l ) ij ∼ N (0 , 1) for every l , where n l is the number of neurons in the l th layer, i e., n 0 = d , n 1 = n 2 = · · · = n L = H .
In the inﬁnite-width limit H → ∞ , the pre-activation f ( l ) =
w ( l ) z ( l − 1) + b ( l ) at every hidden layer tends to an i.i.d.
Gaussian process with covariance Σ ( l − 1) :
R d × R d → R that is deﬁned recursively as
[Equation]
for l = 1 , 2 , . . .
, L .
We also deﬁne
[Equation]
13) where ˙ ϕ is the derivative of ϕ .
The NTK is then expressed as Θ ( L ) ij ( x, x ′ )
=
δ i,j Θ ( L ) ( x, x ′ ) , where
[Equation]
The derivation of this formula is given by Arora et al.
Using the ReLU activation function ϕ ( u )
= max { u, 0 } , we can further calculate Σ ( l ) ( x, x ′ ) and ˙Σ ( l ) ( x, x ′ ) , obtaining
[Equation]
and
[Equation]
For x = x ′ , we obtain Σ ( l ) ( x, x ) =
Σ (0) ( x, x ) + lβ
=
k x k 2 /d
+ ( l + 1) β 2 .
By solving eqs.
(15) and (16) iteratively, the NTK in eq.
(14) is obtained.
B L OCAL STABILITY EXPERIMENT
Instead of considering general perturbations, we here restrict ourselves to local perturbations, z = ± ve ( i ) , where v > 0 is the strength of a perturbation and e ( i ) is the unit vector in i th direction ( i = 1 , 2 , . . . , d ). For each sample x ′ ( µ ) in the test data, if the network does not change its predicted label under any local perturbation for a given v , i.e., arg max j ∈ [ K ] f j ( x ′ ( µ ) ) = arg max j ∈ [ K ] [ f j ( x ′ ( µ ) + αve ( i ) )] for any
i ∈ [ d ] and α ∈ {± 1 } , we say that the network is v -stable at x ′ ( µ ) .
We deﬁne s ( µ ) ( v ) as s ( µ ) ( v ) = 1 if the network is v -stable at x ′ ( µ ) and s ( µ ) ( v ) = 0 otherwise.
The local stability of the network is measured by a quantity
[Equation]
Similarly, the v -stability is deﬁned for the k -local and k -global labels: a given label is said to be v -stable at x ′ ( µ ) if the label does not change under any local perturbation with strength v .
We can also deﬁne s ( v ) for k -local and k -global labels.
In our setting, v = O (1) is enough to change the k -local label, whereas typically v =
˜ O ( √ d ) is needed to alter the k -global label.
where
[Equation]
s ( v ) v depth 1, width 500 depth 7, width 500 2-local 2-global We measure s ( v ) for shallow ( L = 1 and H = 500 ) and deep ( L = 7 and H = 500 ) networks trained on N = 10 4 datasets with random labels by using the SGD of the learning rate η = 0 .and the minibatch size B = 50 . The dimension of input vectors is ﬁxed as d = 100 .
In Fig.
5, s ( v ) is plotted for the shallow and deep networks as well as for the 2-local and 2-global labels.
We observe that s ( v ) in the deep network is smaller than that in the shallow one.
Moreover, s ( v ) in the shallow network is close to that of the 2-global label.
This experimental result indicates that even for learning a completely structureless dataset, leaned features in a deep network tend to be more local compared with those in a shallow one.
C C LASSIFICATION OF I SING SPIN CONFIGURATIONS
In the main text, we have considered quite artiﬁcial dataset (random Gaussian inputs and k -local or k -global label).
Our main result is that local (global) features are more efﬁciently learned by using deeper (shallower) networks.
Here, we demonstrate that this result is relevant to a realistic dataset in physics, i.e., snapshots of Ising spin conﬁgurations.
The Ising model has been studied in physics as a quintessential model for ferromagnets (Nishimori & Ortiz, 2011).
At each site i = 1 , 2 , . . .
, d , a binary spin variable σ i ∈ {± 1 } is assigned, and a spin conﬁguration x is speciﬁed by a set of spin variables, i.e., x = ( σ 1 , σ 2 , . . .
, σ d ) T .
In the one-dimensional Ising model at the inverse temperature β , spin conﬁgurations are generated from a Gibbs distribution,
[Equation]
18) where Z β is a normalization constant known as the partition function in statistical mechanics and
[Equation]
19) is called the Hamiltonian, which corresponds to the energy of the system.
Now we ﬁx two different inverse temperatures β 1 and β 2 , and generate spin conﬁgurations following either P β 1 ( x ) or P β 2 ( x ) with equal probability.
Then we ask whether a given spin conﬁguration x is generated by P β 1 ( x ) or P β 2 ( x ) .
This is a classiﬁcation problem, which can be solved by machine learning.
We consider supervised learning for this classiﬁcation problem, where we ﬁx d = 500 , β 1 = 0 .
1 , and β 2 = 0 .
A training dataset D = { ( x ( µ ) , y ( µ ) : µ = 1 , 2 , . . .
, N } consists of spin conﬁgurations { x ( µ ) } and their labels { y ( µ ) } .
If x ( µ ) is generated at the inverse temperature β 1 (or β 2 ),
[Figure]
The temperature for a spin conﬁguration x is most successfully estimated by measuring its energy, i.e., the value of H ( x ) .
Therefore H ( x ) plays the role of a relevant feature for this classiﬁcation problem.
It is obvious from Eq.
(19) that the energy is a 2-global quantity, and hence, this problem resembles the classiﬁcation of the dataset with the 2-global label.
We then expect that, from our main result discussed in the main text, a shallower network will do better than a deeper network.
Experimental results for a shallow network ( L = 1 and H = 500 ) and a deep one ( L = 7 and
H = 500 ) are shown in Fig.
For small N , both networks show almost identical performance, while for larger N , the shallow network clearly outperforms the deep one.
Apart from this example, thermodynamic quantities in physics are usually written as a sum of local quantities over the entire volume of the system, and hence they are global variables.
Our main result thus implies that shallower networks are more suitable for machine learning of thermodynamic systems.