GKM manifold - definition
Shizuo Kaji and Shintarô Kuroki

1. Introduction
A class of manifold, now referred to as GKM manifold, was ﬁrst discussed in the seminal workby Goresky-Kottwitz-MacPherson in [8] to study the relation between the equivariant cohomology
and the ordinary cohomology. So the letters ‘G’, ‘K’ and ‘M’ stand for the ﬁrst initials of theauthors of [8]. Motivated by their work, the concepts of GKM manifold and GKM graph are
introduced by Guillemin-Zara in [11] to build a bridge between combinatorics and geometry. Thesenotions led to a fertile area of mathematics often called GKM theory which has been studied fromboth of geometry and combinatorics, and also applied to the other areas (e.g. representation
theory, see [6]).
2. Definition
Because GKM theory is studied from several areas, there are diﬀerent versions of deﬁnitions.
Here, we review two major deﬁnitions which are often used in the literature.
The following is the original deﬁnition in [11].
Definition 2.1 ([11]). Let T = T n be an n-dimensional torus, i.e., a commutative, compact,connected n-dimensional Lie group, t be its Lie algebra, and M be a compact 2d-dimensional
manifold with an eﬀective T -action. We say that M is a GKM manifold if it satisﬁes that(1) the ﬁxed point set M T is ﬁnite,(2) the manifold M has a T -invariant almost complex sturcture,(3) and for every p ∈ M T , if n ≥ 2, the weights
αi,p ∈ t∗of the tangential (complex) representation of T on Tp M are pairwise linearly independent,i.e., each pair {αi,p , αj,p } for 1 ≤ i < j ≤ d is linearly independent.
Remark 2.2. When n = 1, a GKM manifold is just the complex projective space CP 1 .
Example 2.3 (toric manifold). A typical example of GKM manifolds is the complx projective
space CP n with the standard T n -action, i.e., every element (t1 , . . . , tn ) ∈ T n acts on a point[z0 : z1 : · · · : zn ] ∈ CP n by
[z0 : z1 : · · · : zn ] 7→ [z0 : t1 z1 : · · · : tn zn ].
In this case, the following (n + 1) points are ﬁxed under the T n -action:(CP n )T = {[1 : 0 : · · · : 0], [0 : 1 : · · · : 0], . . . , [0 : · · · : 0 : 1]}.
Moreover, the weights of each tangential representation of T n on Tp CP n for p ∈ (CP n )T canbe computed as follows (we compute the weights only for n = 2 because it is easy to apply thisThe 2nd author was partially supported by Grant-in-Aid for Scientific Research (S)24224002, Japan Societyfor Promotion of Science.
1computation to the general cases). Let p = [1 : 0 : 0], q = [0 : 1 : 0], r = [0 : 0 : 1] ∈ (CP 2 )T . We
identify the open neighborhood of each ﬁxed point with its tangent space by
Up = {[1 : z1 : z2 ] | zi ∈ C} ≃ {(z1 , z2 ) | zi ∈ C} ≃ Tp CP 2 ,Uq = {[z0 : 1 : z2 ] | zi ∈ C} ≃ {(z0 , z2 ) | zi ∈ C} ≃ Tq CP 2 ,Ur = {[z0 : z1 : 1] | zi ∈ C} ≃ {(z0 , z1 ) | zi ∈ C} ≃ Tr CP 2 .
Then, the induced T 2 -actions on elements in Up , Uq , Ur ⊂ CP 2 are deﬁned by[1 : z1 : z2 ] 7→ [1 : t1 z1 : t2 z2 ],−1[z0 : 1 : z2 ] 7→ [z0 : t1 : t2 z2 ] = [t−11 z0 : 1 : t1 t2 z2 ],−1[z0 : z1 : 1] 7→ [z0 : t1 z1 : t2 ] = [t−12 z0 : t2 t1 z1 : 1],

respectively. Therefore, each tangential (complex) representation decomposes into the following
irreducible representations:
Tp CP 2 ≃ V (α1 ) ⊕ V (α2 );Tq CP 2 ≃ V (−α1 ) ⊕ V (−α1 + α2 );Tr CP 2 ≃ V (−α2 ) ⊕ V (α1 − α2 ),where the symbol V (λ) represents the complex 1-dimensional T -representation space induced fromthe representation λ : T 2 → S 1 ∈ Hom(T, S 1 ) ≃ t∗ and αi is the projection onto the ith coordinate(i = 1, 2). Because α1 and α2 are the basis of t∗ , the weights of each tangential representation arelinearly independent. This establishes that the complex projective space is a GKM manifold.
More generally, a toric manifold X is deﬁned as an n-dimensional, non-singular, complete,complex algebraic variety which has a (C∗ )n -action with dense orbit (see [19] for details), whereC∗ = C \ {0}. If we restrict the algebraic torus (C∗ )n -action on X to the topological (compact)
torus T n -action, then a toric manifold is a GKM manifold.
There is an alternative geometric way to state the third condition in Deﬁnition 2.1. Assume
that M satisﬁes the ﬁrst two conditions in Deﬁnition 2.1. Then, M satisﬁes the third condition
if and only if each connected component of the set of points p ∈ M such that T (p) ≃ S 1 isequivariantly diﬀeomorphic to C \ {0} and its closure is a T -invariant embedded 2-sphere. By thisalternative condition, we can deﬁne the graph Γ by the one-skeleton of M , i.e., the set of pointsp ∈ M such that the orbit T (p) = {p} (a ﬁxed point) or T (p) ≃ S 1 . Namely, the vertices V (Γ) ofΓ are the ﬁxed points of the T -action and the edges E(Γ) of Γ are embedded 2-spheres connecting
two ﬁxed points. With this geometric viewpoint in mind, we give the second deﬁnition of GKM
manifold, which is the generalization of the ﬁrst one.
Definition 2.4 ([9]). Let T be an n-dimensional torus and M be a compact 2d-dimensional
manifold with an eﬀective T -action. We say that M is a GKM manifold if it satisﬁes that theset of the 0-dimensional orbits in M/T is zero dimensional and the set of 1-dimensional orbits inM/T is one dimensional. In other words, the one-skeleton of M has the structure of a graph.
Example 2.5 (torus manifold). A typical example of GKM manifolds in the sense of Deﬁnition
2.4 (but not of Deﬁnition 2.1) is the 2n-dimensional sphere for n ≥ 2 with the standard T n -action,i.e., every element (t1 , . . . , tn ) ∈ T n acts on a point (z1 , . . . , zn , r) ∈ S 2n ⊂ Cn ⊕ R such that|z1 |2 + · · · + |zn |2 + r2 = 1 as follows(z1 , . . . , zn , r) 7→ (t1 z1 , . . . , tn zn , r).
In this case, there are 2 ﬁxed points, i.e., 0-dimensional orbits, which are denoted by(S 2n )T = {(0, . . . , 0, 1), (0, . . . , 0, −1)}.
Moreover, the set of 1-dimensional orbits are the disjoint union of n connected components{(z1 , 0 . . . , 0, r)} ⊔ · · · ⊔ {(0 . . . , 0, zn , r)}2

for r ̸= ±1. Namely, the one-skeleton of S 2n is the graph which has two vertices and n edges whichconnecting these two vertices. Note that S 2 is equivariantly diﬀeomorphic to CP 1 (cf. Example2.3).
More generally, a torus manifold is deﬁned as a 2n-dimensional, compact, oriented manifold
which has an eﬀective T n -action with ﬁxed points. This is deﬁned by Hattori-Masuda in [12] as a
generalization of a unitary toric manifold [16] which is a torus manifold with a T -invariant stablycomplex structure. Every torus manifold is a GKM manifold in the sense of Deﬁnition 2.4 (see[15, 17] for details). In the case of a symplectic (or projective) toric manifold M , the one-skeletonof M is nothing but the one-skeleton of the simple convex polytope which is identiﬁed with M/Tby the moment map.
Example 2.6 (homogeneous space). Let G be a compact, connected Lie group. We call thedimension of a maximal torus of G the rank of G. Let H be a closed, connected, maximal rank
subgroup of G, i.e., a maximal torus T of H is also a maximal torus of G. Then, every homogeneous
space G/H with the standard left T -action is a GKM manifold in the sense of Deﬁnition 2.4 (see [9]for details). If there is a T -invariant almost complex structure on G/H, then such a homogeneous
space is also a GKM manifold in the sense of Deﬁnition 2.1. This condition is equivalent to sayingthat H is a parabolic subgroup of G, that is, G/H is a generalized ﬂag manifold, e.g. the ﬂag
manifold SU (n+1)/T n , the complex Grassmannian U (m+n)/U (m)×U (n), and the 6-dimensional
sphere G2 /SU (3), etc.
3. The Goresky-Kottwitz-MacPherson Theorem
One prominent feature of GKM manifolds is that the T -equivariant cohomology can be described in an algebraic-combinatorial fashion. An equivariantly formal space X deﬁned in [8] is a(possibly singular) space with a compact, connected Lie group G-action such that the Leray-Serre
cohomology spectral sequence for the Borel construction
X ,→ EG ×G X → BG
collapses at E2 -term.
Example 3.1. Every T -space X with H odd (X) = 0 is an equivariantly formal space.
Let X be a (possibly singular) complex, projective, algebraic variety with an algebraic, complex
torus (C∗ )n -action. Suppose that the (C∗ )n -action has ﬁnitely many ﬁxed points, say x1 , . . . , xk ,and ﬁnitely many 1-dimensional orbits, say E1 , . . . , Eℓ . Then, the closure Ej = Ej ∪ {xj0 } ∪ {xj∞ }is an embedded 2-sphere for all j = 1, . . . , ℓ. Let αj0 j∞ ∈ t∗ ≃ H 2 (BT ; R) be the weight of theisotropy T -action on Ej . Recall that H ∗ (BT ; R) is isomorphic to the polynomial ring S(t∗ ) ≃R[α1 , . . . , αn ] generated by degree two elements α1 , . . . , αn .
Theorem 3.2 (Goresky-Kottwitz-MacPherson [8], see also [10]). Let X be a (C∗ )n -spacewith the above conditions. Then, the restricted map HT∗ (X; R) → HT∗ (X T ; R) ≃ ⊕ki=1 H ∗ (BT ; R)
is injective, and its image is the subalgebra{(f1 , . . . , fk ) ∈ ⊕ki=1 H ∗ (BT ; R) | fj0 − fj∞ ∈ (αj0 j∞ ) for 1 ≤ j ≤ ℓ},where (αj0 j∞ ) is the ideal generated by αj0 j∞ ∈ H ∗ (BT ; R).
Remark 3.3. Projective toric manifolds and complex ﬂag manifolds are examples which satisfy
the conditions of this theorem. Moreover, this theorem corresponds to the piecewise polynomial
description [3] and Arabia’s description [1] of the T -equivariant cohomology of those manifolds.
Remark 3.4. We also note that if the above space X is non-singular and we restrict the(C∗ )n -action to the maximal torus T ⊂ (C∗ )n action, then X is a GKM manifold in the sense ofDeﬁnition 2.1. If X is a non-singular algebraic variety with the above (C∗ )n -action, this is called
an algebraic GKM manifold in [18].
Therefore, the equivariant cohomology ring HT∗ (X; R) of the GKM manifold X with the above
conditions can be computed by the one-skeleton of X and the (dual of) isotropy weights for each
Ej . This leads us to deﬁne the labelled graph (Γ, α) from X as follows: Γ is the one-skeleton
3

of X and a label α : E(Γ) → t∗ by α(ej ) = αj0 j∞ for each edge ej corresponding to Ej , whereE(Γ) is the set of oriented edges. This labelled graph (Γ, α) is called a GKM graph in [11] (alsosee [14, 20]). Moreover, in [11], Guillemin-Zara deﬁne an abstract GKM graph (Γ, α) (mightnot be induced from a GKM manifold) and a cohomology ring of GKM graph H(Γ, α) motivatedby Theorem 3.2. Using these notions, they solve the deformation problem of an edge-reﬂecting
polytope ∆ in Rn in [11], i.e., how to deform ∆ so that the directions of its edges are unchanged(also see [2]). This is an application of GKM theory to solve a problem in combinatorics.
Remark 3.5. Let (Γ, α) be a GKM graph (deﬁned in [11]) induced from a GKM manifold inthe sense of Deﬁnition 2.1. Then, the label on each edge e ∈ E(Γ) satisﬁes thatα(e) = −α(ē),
where ē ∈ E(Γ) represents the edge e with its orientation reversed. On the other hand, in [15],Maeda-Masuda-Panov deﬁne a labelled graph induced from the (dual of) isotropy weights of aunitary toric manifold (or more generally an omnioriented torus manifold), called torus graph(also see [5]). Because the label on each edge e ∈ E(Γ) of torus graph (Γ, α) satisﬁesα(e) = ±α(ē),
this gives a slightly generalized GKM graph.
Finally, we also note that Theorem 3.2 for T -manifolds can be generalized to the following
theorem, by using the Chang-Skjelbred lemma [4, Lemma 2.3] (also see [10, 13]) and the MayerVietoris exact sequence of T -equivariant cohomology for each invariant 2-sphere.
Theorem 3.6 (see [7]). Let M be an equivariantly formal, GKM manifold in the sense ofDefinition 2.4. Then, the restricted map HT∗ (M ; Q) → HT∗ (M T ; Q) ≃ ⊕ki=1 H ∗ (BT ; Q) is injective,and its image is the subalgebra{(f1 , . . . , fk ) ∈ ⊕ki=1 H ∗ (BT ; Q) | fj0 − fj∞ ∈ (αj0 j∞ ) for 1 ≤ j ≤ ℓ},where (αj0 j∞ ) ⊂ H ∗ (BT ; Q) is the ideal generated by the dual of isotropy weight of the T -action
on the invariant 2-sphere Ej = Ej ∪ {xj0 } ∪ {xj∞ } (1 ≤ j ≤ ℓ).
References[1] A. Arabia, Cohomologie T -equivariante de la variete de drapeaux d’un groupe de Kac-Moody, Bull. Soc. Math.
France 117 (1989), 129–165. MR1015806 (90i:32042)[2] E. Bolker, V. Guillemin and T. Holm, How is a graph like a manifold? Preprint math.CO/0206103.
[3] M. Brion and M. Vergne, An equivariant Riemann-Roch theorem for complete, simplicial toric varieties, J.
Reine Angew. Math. 482 (1997), 67–92. MR1427657 (98a:14067) Zbl 0862.14006.
[4] T. Chang and T. Skjelbret, The topological Schur lemma and related result, Ann. Math., 100 (1974), 307–321.
[5] A. Darby, Torus manifold in equivariant complex bordism, arXiv: 1409.2720.
[6] P. Fiebig, Lusztig’s conjecture as a moment graph problem, Bull. Lond. Math. Soc. 42 (2010), no. 6, 957–972.
[7] M. Franz and V. Puppe, Exact sequences for equivariantly formal spaces, C. R. Math. Acad. Sci. Soc. R. Can.
33 (2011), 1–10
[8] M. Goresky, R. Kottwitz and R. MacPherson, Equivariant cohomology, Koszul duality, and the localization
theorem, Invent. Math. 131 (1998), 25–83. MR1489894 (99c:55009), Zbl 0897.22009[9] V. Guillemin, T. Holm and C. Zara, A GKM description of the equivariant cohomology ring of a homogeneousspace, J. Algebraic Combin. 23 (2006) no. 1, 21–41.
[10] V. Guilleminn, S. Sternberg, Supersymmetry and equivariant de Rham theory, Springer Berlin, 1999.
[11] V. Guillemin and C. Zara, One-skeleta, Betti numbers, and equivariant cohomology, Duke Math. J. 107, 2(2001), 283–349.
[12] A. Hattori and M. Masuda, Theory of multi-fans, Osaka J. Math. 40 (2003), 1–68.
[13] W. Y. Hsiang, Cohomology theory of topological transformation groups, Ergebnisse der Mathematik und ihrer
Grenzgebiete, Band 85. Springer-Verlag, New York-Heidelberg, 1975.
[14] S. Kuroki, Introduction to GKM theory, Trends in Math. Vol. 11 No. 2, 113–129 (2009).
[15] H. Maeda, M. Masuda, T. Panov, Torus graphs and simplicial posets, Adv. Math. 212 (2007), 458–483.
[16] M. Masuda, Unitary toric manifolds, multi-fans and equivariant index. Tohoku Math. J. 51 (1999), 237–265.
[17] M. Masuda and T. Panov, On the cohomology of torus manifolds, Osaka J. Math. 43 (2006), 711–746.
[18] C-C. M. Liu and A. Sheshmani, Equivariant Gromov-Witten invariants of algebraic GKM manifolds, arXiv:1407.1370.
[19] T. Oda, Convex Bodies and Algebraic Geometry. An Introduction to the Theory of Toric Varieties, Ergeb.
Math. Grenzgeb. (3), 15, Springer-Verlag, Berlin, 1988.
4

[20] J. S. Tymoczko, An introduction to equivariant cohomology and homology, following Goresky, Kottwitz, andMacPherson, Snowbird lectures in algebraic geometry, Contemp. Math., 388, Amer. Math. Soc., Providence,
RI (2005), 169–188.
Department of Mathematical Sciences, Faculty of Science, Yamaguchi University, 1677-1, Yoshida,Yamaguchi, 753-8512, Japan
E-mail address: skaji@yamaguchi-u.ac.jp
Graduate School of Mathematical Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku,Tokyo, 153-8914, Japan
E-mail address: kuroki@ms.u-tokyo.ac.jp

5