Lew's extremal characterization conjecture for simplicial complexes

From papers

Let XX be a simplicial complex on vertex set VV of size nn, with h(X)=dh(X)=d. Write μk(X)\mu_k(X) for the parameter appearing in the equality below, and let Δm\Delta_m denote the complete simplicial complex on m+1m+1 vertices; Δd(d1)\Delta_d^{(d-1)} is its (d1)(d-1)-dimensional skeleton. The symbol * denotes the join of simplicial complexes, and XYX\cong Y means that XX and YY are isomorphic simplicial complexes. Lew's conjecture. If

μk(X)=(d+1)(k+1)dn\mu_k(X)=(d+1)(k+1)-dn

for some kk, then

X(Δd(d1))(nk1)Δ(d+1)(k+1)dn1.X\cong \left(\Delta_d^{(d-1)}\right)^{*(n-k-1)}*\Delta_{(d+1)(k+1)-dn-1}.

In particular, dim(X)=k\dim(X)=k. Lew proposed this as a characterization of the simplicial complexes attaining equality in the relevant spectral-gap lower bound; the supplied source does not state whether the conjecture has been resolved.

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Primary source

Xiongfeng Zhan, Xueyi Huang and Huiqiu Lin, “Proof of Lew's conjecture on the spectral gaps of simplicial complexes”, arXiv:2407.10398 (2024).

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