arXiv:1908.07340v2 [math.DG] 28 Sep 2019 eigasyPopc,156 ocw usa -aladdr E-mail Russia, Moscow, 125468 Prospect, Leningradsky fteRsinAaeyo cecs 0 seih tet 12 street, Usievicha 20, Sciences, of [email protected] Academy address: Russian the of -alades [email protected] address: E-mail clrcurvature scalar i aifisRc= Ric satisfies Ric ( Keywords soeo h ujc forresearch. our of clas condit subject to curvature the suitable related of monograp a facts one satisfying interesting the is some in Einstein here compact summarized present of We all been topology [1]. have and Besse results A. curvature differ geomet Preliminary among between Riemannian big relationships in much manifolds. history finding been long has in a there ometers history, has whole manifolds the Einstein Throughout of study The 1 MSC2010: number. nteBtiadTciaanmeso compact of numbers Tachibana and Betti the On ,g M, ∗ ‡ † A eateto aaAayi n iaca ehoois Fi Technologies, Financial and Analysis Data of Department eateto ahmtc,RsinIsiuefrScientifi for Institute Russian Mathematics, of Department eateto ahmtc,Uiest fHia on Carm Mount Haifa, of University Mathematics, of Department ldmrRovenski Vladimir ealta an that Recall . ssi obe to said is ) Introduction n h oooyo isenmnfls nteppr rt w first, paper, ( the betwe In Einstein relationships manifolds. compact Einstein the of finding topology the in and interest great shown have npriua,( particular, in etoa curvatures sectional oia peewe h iiu fisscinlcurvatures sectional its of minimum the when sphere logical oteaoeoe)frTciaanmeso opc Einstei compact a of numbers Tachibana ( for ones) above the to ,g M, hogottehsoyo isenmnfls differential manifolds, Einstein of history the Throughout with ) isenmnfl,scinlcraue et ubr Tachiban number, Betti curvature, sectional manifold, Einstein : 32;5C3 53C44 53C43; 53C20; s < α hrfr,ayEnti aiod( manifold Einstein any Therefore, . isenmanifold Einstein g α ,g M, 0. oevr ndmnin ehv i ( = Ric have we dimensions in moreover, ; n sashrclsaefr hntemnmmo its of minimum the when form space spherical a is ) dmninl( -dimensional isenmanifolds Einstein α/n > egyStepanov Sergey , ∗ eod epoetopooiin (similar propositions two prove we Second, . ,g M, Abstract ihEnti constant Einstein with ) with n 1 ≥ isenconstant Einstein )cnetdReana manifold Riemannian connected 2) † n rn Tsyganok Irina and s:[email protected] ess: ,g M, 10Mso,Rsi,E-mail Russia, Moscow, 5190 n ehia Information Technical and c ac nvriy 49-55, University, nance fdmnin and 2 dimensions of ) > α l af,395 Israel, 31905, Haifa, el, α rv hta that prove e ncurvature en α/ > sahomo- a is 0 fisRcitensor Ricci its if manifold n geometers s/n ( n icto of sification 2); + o,which ion, ) nilge- ential Einstein g o its for by h ‡ ry. a 3 is a space form. However, the study of Einstein manifolds becomes much more complicated in dimension four and higher (see [1, p. 44]). B. An interesting problem in differential geometry is to determine whe- ther a smooth manifold M admits an Einstein metric g. When α > 0, the well-known example are symmetric spaces, which include the sphere Sn(1) with Einstein constant α = n 1 and sectional curvature sec = 1, the product of two spheres Sn(1) Sn(1) with− Einstein constant α = n 1 and 0 sec 1, the complex projective× space CP m = S2m+1/ S1 with Fubini-Study− metric,≤ ≤ Einstein constant α = 2m +2 and 1 sec 4 (see [2, pp. 86, 118, 149-150]). Recall that if (M,g) is a compact≤ Einstein≤ manifold with curvature bounds of the type 3n/(7n 4) < sec 1, then (M,g) is isometric to a spherical space form. This is− probably not≤ the best possible bound. For example, for n = 4 the sharp bound is 1/4 (see [1, p. 6]). In both these cases, the manifolds are real homology spheres (see [3, p. XVI]) and, therefore, any such manifold (M,g) has the homology groups of n-sphere. In particular, its Betti numbers are b1 (M)= ... = bn−1(M) = 0. C. We can state that a basic problem in Riemannian geometry was to classify Einstein 4-manifolds with positive or nonnegative sectional curvature in the categories of either topology, diffeomorphism, or isometry (see, for example, [4, 5, 6, 7] etc.). It was conjectured that an Einstein four manifold with α> 0 and non-negative sectional curvature must be either S4, CP2, S2(1) S2(1) or quotient. For example, it was proved that if the maximum of the se×ctional curvatures of a compact Einstein 4-manifold is bounded above by (2/3) α, or if α = 1 and the minimum of the sectional curvatures (1/6)(2 √2), then the manifold is isometric to S4, RP4 or CP2 (see [6]). On≥ the other hand,− the author of the paper [7] obtained classification of four-dimensional complete Einstein manifolds with positive Einstein constant and pinched sectional curvature. Consider this problem from another side. Given a (M,g), the notion of symmetric curvature operator R¯ :Λ2M Λ2M acting on the space of 2-forms Λ2M is an important invariant of Riemannian→ metric (see [2, p. 83]; [8, 9]). A famous theorem of Tachibana (see [10]) asserts that a compact Einstein manifold (M,g) with positive curvature operator R¯ is a spherical space form. In general, if (M,g) is a compact Riemannian manifold with positive curvature operator R¯, then it is a spherical space form (see [11]). ◦ 2 2 Let R : S0 M S0 M be the symmetric curvature operator of the second → 2 kind acting on the space of traceless symmetric 2-tensors S0 M (see [1, p. 52]; [9, 13]). In turn, Kashiwada proved that a compact Einstein manifold (M,g) ◦ with positive curvature operator R is a spherical space form (see [9]). This statement is an analogue of the famous theorem of Tachibana from [10]. But it is less known, than Tachibana’s theorem. In contrast, if a Riemannian manifold (M,g) is complete and satisfies sec δ > 0, then (M,g) is only compact with diam (M,g) π/√δ (see [2, p. 251]).≥ ≤ Remark 1. From [2, Theorem 10.3.7] we can conclude that there are many manifolds that have metric of positive or nonnegative sectional curvature but do not admit metric with nonnegative curvature operator R¯ (see also [2, p. 352]).

2 In particular, for three-dimensional manifolds the condition sec > 0 is equivalent to the condition R>¯ 0 (see [9]). Using Kashiwada’s theorem from [9] we can prove the following theorem. Theorem 1.1. Let (M,g) be a compact connected Einstein manifold with pos- itive Einstein constant α and let δ be the minimum of its positive sectional curvature. If δ > α/n, then (M,g) is a spherical space form. We can present the generalization of above result in the following form. Theorem 1.2. Let (M,g) be a compact connected Einstein manifold with pos- itive Einstein constant α and let δ be the minimum of its positive sectional curvature. If δ >α/(n + 2), then (M,g) is a homological sphere. Obviously, Sn(1) Sn(1) is not an example for Theorem 1.1 because the minimum of its sectional× curvature is zero and α = n 1. On the other hand, we know that the CPm is an− Einstein manifold with Einstein constant α =2m + 2 and sectional curvature bounded below by δ = 1. Then the inequality α < (n + 2) δ can be rewritten in the form 1 < δ because n =2m. Therefore, CPm is not an example for Theorem 1.1. On the other hand, we know from [2, p. 328]) that all even dimensional Riemannian manifolds with positive sectional curvature have vanishing odd di- mensional homology groups. Thus, Theorem 1.1 complements this statement. Let (M,g) be an n-dimensional compact connected Riemannian manifold. Denote by ∆(p) the Hodge Laplacian acting on differential p-forms on M for p = 1,...,n 1. The spectrum of ∆(p) consists of an unbounded sequence of nonnegative− eigenvalues which starts from zero if and only if the p-th Betti number bp(M) of (M,g) does not vanish (see [14]). The sequence of positive eigenvalues of ∆(p) is denoted by

0 < λ(p) <...<λ(p) < . . . + . 1 m → ∞

In addition, if Fp(ω) σ> 0 (see definition (4) of Fp) at every point of M, then ≥ λ(p) σ (see [14, p. 342]). Using this and Theorem 1.1, we get the following. 1 ≥ Corollary 1.1. Let (M,g) be a compact connected Einstein manifold with pos- itive Einstein constant α and sectional curvature bounded below by a constant (p) δ > 0 such that δ > α/(n + 2). Then the first eigenvalue λ1 of the Hodge Laplacian ∆(p) satisfies the inequality λ(p) (1/3) ((n + 2) δ α) (n p). 1 ≥ − − Remark 2. In particular, if (M,g) is a Riemannian manifold with curvature operator of the second kind bounded below by a positive constant ρ > 0, then using the main theorem from [22], we conclude that λ(p) ρ (n p). 1 ≥ − Conformal Killing p-forms (p =1,...,n 1) have been defined on Rieman- nian manifolds more than fifty years ago by S.− Tachibana and T. Kashiwada (see [15, 16]) as a natural generalization of conformal Killing vector fields. The vector

3 space of conformal Killing p-forms on a compact Riemannian manifold (M,g) has finite dimension tp(M) named the Tachibana number (see e.g. [17, 18, 19]). Tachibana numbers t1(M),...,tn−1(M) are conformal scalar invariant of (M,g) satisfying the duality condition tp(M)= tn−p(M). The condition is an analog of the well known Poincar´eduality for Betti numbers. Moreover, we proved in [18, 19] that Tachibana numbers t1(M),...,tn−1(M) are equal to zero on a compact Riemannian manifold with negative curvature operator or negative curvature operator of the second kind. We prove the following theorem, which is an analog of Theorem 1.1. Theorem 1.3. Let (M,g) be an Einstein manifold with sectional curvature bounded above by a negative constant δ < 0 such that δ > α/(n + 2) for the − − Einstein constant α. Then Tachibana numbers t1(M),...,tn−1(M) are equal to zero.

2 Proof of results

Let (M,g) be an n-dimensional (n 2) Riemannian manifold and let Rijkl and ≥ Rij be, respectively, the components of the Riemannian curvature tensor R and the Ricci tensor Ric in orthonormal basis e1,...,en of TxM at an arbitrary point x M. We consider an arbitrary symmetric{ 2-tensor} ϕ on (M,g). At any point∈ x M, we can diagonalize ϕ with respect to g, using orthonormal ∈ basis e ,...,en of TxM. In this case, the local components of ϕ have the { 1 } form ϕij = λi δij . Let sec(ei,ej) be the sectional curvature of the plane of TxM generated by ei and ej. We can express sec(ei, ej ) in terms of R in the following form (see [1, p. 436]; [23]):

1 2 ik jl ik j sec(ei,ej ) (λi λj ) = Rijlkϕ ϕ + Rij ϕ ϕ (1) 2 X i=6 j − k If (M,g) is an Einstein manifold and its sectional curvature satisfies the condi- tion sec δ for a positive constant δ, then from (1) we obtain the inequality ≥ ik jl s ik 1 2 Rijlk ϕ ϕ + ϕ ϕik δ (λi λj ) . (2) n ≥ 2 X i=6 j − 2 If traceg ϕ = i λi = 0, then the identity holds i (λi ) = 2 i

1 2 2 2 2 (λi λj ) = (n 1) (λi) 2 λi λj = n (λi ) = n ϕ . 2 X − − X − X X k k i=6 j i i

ik jl s ik 2 Rijlkϕ ϕ + ϕ ϕik nδ ϕ . (3) n ≥ k k From (3) we obtain the inequality

ik jl 2 Rijlkϕ ϕ (nδ α) ϕ . ≥ − k k

4 ◦ Then R > 0 for the case when α

i i2... ip j p 1 ij i3... ip kl Fp(ω)= Rij ω ω − Rijkl ω ω (4) i2...ip − 2 i2... ip for the components ωi1...ip = ω( ei1 ,...,eip ) of an arbitrary differential p-form ω. If the quadratic form Fp(ω) is positive definite on a compact Riemannian mani- fold (M,g), then the p-th Betti number of the manifold vanishes (see [20, p. 61]; [3, p. 88]). At the same time, in [21] the following inequality Fp(ω) ≥ p (n p) ε ω 2 > 0 was proven for any nonzero p-form ω on a Riemannian manifold− withk k curvature operator R¯ ε > 0. On the other hand, in [22] the 2 ≥ inequality Fp(ω) p(n p) δ ω > 0 was proved for any nonzero p-form ω ≥ − k k ◦ on a Riemannian manifold with curvature operator R δ > 0. In these cases, ≥ b1(M),...,bn−1(M) are equal to zero (see [20]). We can improve these results for the case of Einstein manifolds. First, we will prove the following lemma. Lemma 2.1. Let (M,g) be an Einstein manifold with Einstein constant α and sectional curvature bounded below by a positive constant δ > 0 such that α < 2 (n + 2)δ. Then Fp(ω) (1/3)((n + 2) δ α)(n p) ω > 0 for any nonzero p-form ω and an arbitrary≥ 1 p n 1−. − k k ≤ ≤ − Proof. Let p [n/2], then we can define the symmetric traceless 2-tensor ≤ ϕ(i1i2...ip) with local components (see [22]) p (i1i2...ip) 2p ϕ = ωi ...i − ji ...i gki + ωi ...i − ki ...i gji gjk ωi ...i jk X 1 a 1 a+1 p a 1 a 1 a+1 p a  − n 1 p a=1 for each set of values of indices ( i1 i2 ...ip) such that 1 i1

il (i1...ip) jk 2(n +4p) i i2...ip j Rijkl ϕ ϕ = p Rij ω ω (i1...ip) n i2...ip

ij i3...ip kl 4p 2 3 (p 1) Rijkl ω ω s ω ; (5) − − i3...ip − n2 k k  2p(n +2)(n p) ϕ¯ 2 = − ω 2 , (6) k k n k k 2 ik jl (i1...ip) (j1...jp) where ϕ¯ = g g gi j ...gi j ϕ ϕ and k k 1 1 p p ij kl ω 2 = ωi1i2...ip ω = gi1j1 ...gipjp ω ω k k i1i2...ip i1...ip j1...jp ij −1 for g = (g )ij . If(M,g) is an Einstein manifold, then formulas (4) and (5) s 2 p−1 ij i3...ip kl can be rewritten in the form Fp(ω)= ω Rijkl ω ω and n k k − 2 i3...ip

il(i1 ...ip) jk 2n +4p 2 ij i3...ip kl Rijklϕ ϕ = p s ω 3(p 1)Rijkl ω ω . (i1...ip)  n2 k k − − i3...ip  (7)

5 On the other hand, for a fixed set of values of indices (i1,i2,...,ip) such that 1 i

il (i1...ip) jk s 2 Rijkl ϕ ϕ nδ ϕ¯ . (9) (i1...ip) ≥  − n k k Using (9) we deduce from (7) the following inequality:

s 2 6p Fp(ω) nδ ϕ¯ . (10) ≥  − n +2  k k In conclusion, using (6) we can rewrite the inequality (10) in the following form: 2 Fp(ω) (1/3) ((n + 2) δ α) (n p) ω . (11) ≥ − − k k It is obvious that if the sectional curvature of an Einstein manifold (M,g) satisfy the condition sec δ for a positive constant δ, then the of (M,g) satisfies the≥ inequality s n(n 1) δ> 0. In this case, if (n 1) δ α< ≥ − − ≤ (n + 2) δ, then from (11) we deduce that the quadratic form Fp(ω) is positive definite for any p [n/2]. In turn, we know from [24] that Fp(ω)= Fn−p( ω) ≤ ∗ and ω 2 = ω 2 for any p-form ω (1 p n 1) and the well-known Hodgek stark operatork∗ k :ΛpM Λn−pM acting≤ ≤ on the− space of p-forms ΛpM. Therefore, the inequality∗ (11)→ holds for any p = 1,...,n 1. The lemma is proved. −  It is known that if for any smooth p-form ω and an arbitrary p =1,...,n − 1 the quadratic form Fp(ω) is positive definite on an n-dimensional compact Riemannian manifold (M,g), then the Betti numbers b1(M),...,bn−1(M) of the manifold vanish (see [3, p. 88]; [14, pp. 336-337]). In this case, Theorem 1.2 is automatically deduced from Lemma 2.1. If the curvature of an Einstein manifold (M,g) satisfies condition sec δ < 0 for a positive constant δ, then the Einstein constant of (M,g) satisfies≤ −the satisfies the obvious inequality α (n 1) δ< 0. On the other hand, from ik ≤−jl − 2 (1) we deduce the inequality Rijlkϕ ϕ ( nδ + α) ϕ . Therefore, if δ > ◦ ≤− k k α/n, then R < 0. In this case, the Tachibana numbers t1(M),...,tn−1(M) are− equal to zero (see [19]). We proved the following proposition. Proposition 2.1. Let (M,g) be an Einstein manifold with sectional curvature bounded above by a negative constant δ < 0 such that δ > α/n for the − − Einstein constant α. Then its Tachibana numbers t1(M),...,tn−1(M) are equal to zero. We can complete this result. If an Einstein manifold (M,g) satisfies the curvature condition sec δ < 0 for a positive constant δ, then from (3) and ≤ − 2 (7) we deduce the inequality Fp(ω) 1/3 ((n + 2) δ + α)(n p) ω for any ≤− − k k p = 1,...,n 1. Therefore, the Tachibana numbers t1(M),...,tn−1(M) of a compact Einstein− manifold (M,g) with sectional curvature bounded above by a negative constant δ such that δ α/(n +2) are equal to zero. − ≥−

6 References

[1] Becce A., Einstein manifolds, Springer-Verlag, Berlin, 1987. [2] Petersen P., Riemannian Geometry, Springer Science, New York, 2016. [3] Goldberg S. I., Curvature and Homology, Dover Publications Inc., 1998. [4] Yang D., Rigidity of Einstein 4-manifolds with positive curvature, Invent. Math., 142, 435–450 (2000). [5] Wu P., Curvature decompositions on Einstein four-manifolds, New York Journal of Mathematics, 23, 1739–1749 (2017). [6] Ezio´ A.C., On Einstein four-manifolds, J. Geom. Phys. 51(2), 244-255 (2004). [7] Cao X., Tran H., Einstein four-manifolds of pinched sectional curvature, Advance in Mathematics, 335 (7), 322–242 (2018). [8] Bourguignon J.P., Karcher H., Curvature operators: pinching estimates and geometric examples, Ann. Sci. Ec. Norm. Sup., 11, 71–92 (1978). [9] Kashiwada T., On the curvature operator of the second kind, Natural Science Report, Ochanomozu University, 44 (2), 69–73 (1998). [10] Tachibana Sh., A theorem on Riemannian manifolds with positive curva- ture operator, Proc. Japan Acad., 50, 301–302 (1974). [11] B¨ohm C., Wilking B., Manifolds with positive curvature operators are space forms, Ann. of Math., 167, 1079–1097 (2008). [12] Micalleff M., Moore J.D., Minimal two-sphere and topology of manifolds with positive curvature on totally isotropic two-planes, Ann. of Math., 127, 199–227 (1988). [13] Nishikawa S., On deformation of Riemannian metrics and manifolds with positive curvature operator, pp. 202–211. In “Curvature and Topology of Riemannian Manifolds”, Springer, London, 1986. [14] Chavel I., Eigenvalues in Riemannian Geometry, Academic Press. INC, Orlando, 1984. [15] Kashiwada T., On conformal Killing tensor, Natural. Sci. Rep. Ochano- mizu Univ., 19 (2), 67–74 (1968). [16] Tachibana S., On conformal Killing tensor in a Riemannian space, Tohoku Math. Journal, 21, 56–64 (1969). [17] Stepanov S.E., MikeˇsJ., Betti and Tachibana numbers of compact Rie- mannian manifolds, Differential Geometry and its Applications, 31 (4), 486–495 (2013).

7 [18] Stepanov S. E., Curvature and Tachibana numbers, Mat. Sb., 202 (7), 135–146 (2011). [19] Stepanov S.E., Tsyganok I.I., Theorems of existence and non-existence of conformal Killing forms, Russian Math. (Iz. VUZ), 58 (10), 46–51 (2014). [20] Bochner S., Yano K., Curvature and Betti numbers, Princeton, 1953. [21] Meyer D., Sur les vari´et´es riemanniennes ´aoperateur de courbure positif, C. R. Acad. Sc., Paris, 272, 482–485 (1971). [22] Tachibana S., Ogiue K., Les vari´et´es riemanniennes dont l’op´erateur de coubure restreint est positif sont des sph`eres d’homologie r´eelle, C. R. Acad. Sc. Paris, 289, 29–30 (1979). [23] Berger M., Ebin D., Some decompositions of the space of symmetric ten- sors on a Riemannian manifold, Journal of Differential Geometry, 3, 379– 392 (1969). [24] Kora M., On conformal Killing forms and the proper space of ∆ for p- forms, Math. J. Okayama University, 22 (2), 195–204 (1980).

8