Remarks on Missing Faces and Generalized Lower Bounds on Face Numbers Eran Nevo¤ April 15, 2009 Dedicated to Anders BjÄorneron the occasion of his 60th birthday. Abstract We consider simplicial polytopes, and more general simplicial com- plexes, without missing faces above a ¯xed dimension. Sharp analogues of McMullen's generalized lower bounds, and of Barnette's lower bounds, are conjectured for these families of complexes. Some partial results on these conjectures are presented. 1 Introduction For simplicial polytopes, McMullen's generalized lower bounds on face numbers were proved by Stanley, conveniently phrased as nonnegativity of the corre- sponding g-vector [16]. As the matrix that sends the g-vector to its f-vector has nonnegative entries, Stanley's result immediately implies Barnette's lower bound theorem for simplicial polytopes. In turn, Barnette's result immediately implies that the simplex minimizes all face numbers among simplicial polytopes with the same dimension. We will phrase conjectures analogous to these three results, depending on a new parameter, namely the maximal size of a missing face, i.e. the maximal size of a minimal non face with respect to inclusion. This gives a hierarchy of conjectures on lower bounds on face numbers, interpolating between the gen- eralized lower bound conjecture for simplicial spheres [12] and Gal's conjecture for flag spheres [8]. We will now work our way to these three conjectures, from weakest to strongest. It is well known, and easy to prove, that among all simplicial complexes with a nonzero (reduced) d-homology, the boundary of the (d+1)-simplex minimizes all face numbers. Similarly, Meshulam proved that among all flag complexes with a nonzero (reduced) d-homology, the boundary of the (d + 1)-dimensional crosspolytope minimizes all face numbers [13]. (Recently Athanasiadis [1] proved that for the subfamily of flag homology d-spheres the h-vector is minimized by ¤Department of Mathematics, Cornell University, Ithaca USA, E-mail address: er- [email protected]. Research partially supported by an NSF Award DMS-0757828. 1 the boundary of the (d + 1)-dimensional crosspolytope, hence so are the face numbers.) For this minimization problem we can clearly assume that the complexes are d-dimensional, as restricting to the d-skeleton cannot make the d-th homology vanish and cannot increase the face numbers. We ¯nd it natural to view these two families of simplicial complexes as extreme cases of the following families. Let C(i; d) be the family of d-dimensional simplicial complexes with a nonzero reduced d-homology (if ¢ 2 C(i; d) then H~d(¢; Z) 6= 0) and with no missing faces of dimension > i.(F is a missing face of ¢ if its boundary @F ⊆ ¢ and F2 = ¢. Its dimension is jF j ¡ 1.) Thus, C(1; d) are the flag d-complexes with nonzero d-th homology and C(d + 1; d) are all the d-complexes with a nonzero d-th homology. (Clearly if i > d + 1 then C(i; d) = C(d + 1; d).) Denote by fi(¢) the number of i-dimensional faces in the complex ¢. Let d ¸ 0; 0 < i be integers. Then there exist unique integers q ¸ 0; 1 · r · i such that d + 1 = qi + r. (Note that the range 1 · r · i is unusual. It will simplify the writing later on.) Let S(i; d) := @σi ¤ ::: ¤ @σi ¤ @σr; where @σi, the boundary of the i-simplex, appears q times in this join. Then S(i; d) is a d-dimensional simplicial sphere. Inspired by Meshulam [13], we prove that it has the following extremal properties: Theorem 1.1. Let d ¸ 0; 0 < i · d + 1 be integers. Write d + 1 = qi + r where 1 · r · i and q; r are integers. Let ¢ 2 C(i; d). Then: (a) If i divides d + 1 then fj(¢) ¸ fj(S(i; d)) for every j. (b) For any i, f0(¢) ¸ f0(S(i; d)). (c) For any i, fj(¢) ¸ fj(S(i; d)) for every 1 · j · r. (d) If i divides d + 1 and fj(¢) = fj(S(i; d)) for every j then ¢ = S(i; d). The cases i = d + 1 and i = 1 recover the two known results mentioned above. We write (b) and (c) separately on purpose, as (b) will play a special role. The condition i j (d + 1) in part (a) seems to be an artifact of the proof. Conjecture 1.2. Let d ¸ 0 and 0 < i · d + 1 be integers. Let ¢ 2 C(i; d). Then fj(¢) ¸ fj(S(i; d)) for every j. Moreover, if equality is attained for every j then ¢ = S(i; d). A re¯ned question is to give lower bounds on face numbers of complexes in C(i; d) with a given number of vertices. The answer to this question for simplicial polytopes is a well known result by Barnette, often referred to as `The lower bound theorem' [3]. Barnette later showed that these lower bounds hold for all triangulated manifolds [2]. Kalai showed they hold for all homology spheres, and more general complexes, and characterized the case of equality [10]. To state these results, we de¯ne stacked polytopes and homology spheres. A stacking is the operation of adding a pyramid over a facet of a given sim- plicial polytope. A polytope is stacked if it can be obtained from a simplex 2 by repeating the stacking operation ¯nitely many times. Let Sk(d; n) be the boundary complex of a stacked (d + 1)-polytope with n vertices. While the combinatorial type of Sk(d; n) is not unique, its face numbers are determined. Next, a d-dimensional complex ¢ is a homology sphere if for every face F in ¢ (including the empty set), and for every 0 · j, there is an isomorphism of » dim(¢)¡jF j m reduced homology groups H~j(lk(F; ¢); Z) = H~j(S ; Z) where S de- notes the m-dimensional sphere, Z the integers and lk(F; ¢) is the link of F in ¢. In particular, the boundary complex of a simplicial polytope is a homology sphere; however there are many non-polytopal examples of homology spheres, e.g. [11]. The following is the lower bound theorem (LBT): Theorem 1.3. ([2, 3] and [10]) Let d ¸ 3, and let ¢ be the boundary complex of a simplicial (d + 1)-polytope, or more generally a homology d-sphere, with n vertices. Then fj(¢) ¸ fj(Sk(d; n)) for every j. If equality holds for some j ¸ 1 then ¢ is combinatorially isomorphic to some Sk(d; n). We now seek an analogue of this result when an upper bound on the dimen- sion of missing faces is speci¯ed. Let HS(i; d; n) be the family of d-dimensional homology spheres with n vertices and without missing faces of dimension > i. Let d ¸ 0; 0 < i and d+1 = qi+r as before (1 · r · i). If HS(i; d; n) 6= ;, then by Theorem 1.1(b), n ¸ q(i + 1) + (r + 1). Hence the following de¯nition makes sense: S(i; d; n) := @σi ¤ ::: ¤ @σi ¤ Sk(r ¡ 1; n ¡ q(i + 1)), where @σi appears q times in this join. This is possible unless r = 1 and n > q(i + 1) + 2. In the later case de¯ne S(i; d; n) = @σi ¤ ::: ¤ @σi ¤ Sk(i; n ¡ (q ¡ 1)(i + 1)), where @σi appears q ¡ 1 times in this join. In any case, S(i; d; n) 2 HS(i; d; n). Conjecture 1.4. If ¢ 2 HS(i; d; n) then fj(¢) ¸ fj(S(i; d; n)) for every j. Clearly Conjecture 1.4 implies Conjecture 1.2 restricted to homology spheres. For i = d + 1 and i = d the assertion of the conjecture is the LBT, Theorem 1.3. For i = d ¡ 1 the conjecture holds if ¢ is the boundary of a simplicial polytope, and follows from the celebrated g-theorem [4, 16]. Here Stanley's result is used. Surprisingly, in this case equalities for all j's in the conjecture imply that ¢ = S(d ¡ 1; d; n). This follows from recent results in [15]. For i = 1, the conjectured lower bounds (for flag homology spheres) would follow from Gal's conjecture on the γ-polynomial [8, Conjecture 2.1.7]. In this case there are many examples of equalities for all j's in the conjecture. We now relate the g-theorem and Gal's conjecture, by de¯ning new `g- vectors' suitable for the families of d-dimensional homology spheres without missing faces of dimension > i, denoted by HS(i; d). Let d ¸ 0 and i > 0 be integers, and let q ¸ 0; 1 · r · i be the unique integers such that d + 1 = qi + r. For such d and i de¯ne the polynomial i q r Pd;i(t) := (1 + t + ::: + t ) (1 + t + ::: + t ): It is symmetric as a multiplication of symmetric polynomials. Further, denote P¡1;i := 1 (a constant polynomial). De¯ne the ordered set of polynomials 2 b d+1 c Bd;i := (Pd;i(t); tPd¡2;i(t); t Pd¡4;i(t); :::; t 2 P d+1 (t)): d¡2b 2 c;i 3 Note that Bd;i is a basis for the space of symmetric polynomials of degree at d+1 most d + 1 and axis of symmetry at `degree' 2 (over the rationales, say). For (d;i) (d;i) (d;i) a symmetric polynomial h(t) in this space, let g (h(t)) = (g0 ; :::; g d+1 ) b 2 c be the vector of coe±cients in the expansion of h(t) in the basis Bd;i.