Optimal torus manifold conjecture for multifans

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Let XX range over real 2n2n-dimensional torus manifolds corresponding to a multifan SX\mathcal S_X. Let hk(SX)=∑i=kn(−1)i−k(ik)en−i(SX)h_k(\mathcal S_X)=\sum_{i=k}^n(-1)^{i-k}\binom{i}{k}e_{n-i}(\mathcal S_X). Optimal torus manifold conjecture. Among the torus manifolds corresponding to SX\mathcal S_X, there exists an optimal torus manifold whose Tn\mathbb T^n-equivariant Betti numbers are minimally non-negative and satisfy

b2k(X)≥hk(SX),k=0,1,…,n,b_{2k}(X)\geq h_k(\mathcal S_X),\qquad k=0,1,\ldots,n,

with minimal values obeying Poincaré duality b2n−r≤brb_{2n-r}\leq b_r, the Hard Lefschetz theorem, and consistency with all Tn\mathbb T^n-equivariant characteristic classes encoded by SX\mathcal S_X. The proposal seeks a preferred torus manifold realizing the smallest admissible Betti data compatible with the multifan; the source gives no resolution.

References

Primary source

Per Berglund and Tristan Hübsch, “Chern Characteristics and Todd-Hirzebruch Identities for Transpolar Pairs of Toric Spaces”, arXiv:2403.07139 (2026).

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