Cell-complex realization conjecture for spheres with gk=1g_k=1

Let k2k\geq 2, d2kd\geq 2k, and let Δ\Delta be a (d1)(d-1)-sphere with gk(Δ)=1g_k(\Delta)=1. Cell-complex realization conjecture. There exists a dd-dimensional cell complex C\mathcal{C} such that all faces of C\mathcal{C} of dimension at most dkd-k are faces of Δ\Delta, all faces except one dd-face are simplices, and the exceptional dd-face is a homology dd-ball whose boundary is a sphere in S(dk,d1)S(d-k,d-1) with gk=1g_k=1. Every two faces of C\mathcal{C} intersect in a common, possibly empty, face; the geometric realization of C\mathcal{C} is a homology dd-ball; and its boundary complex is Δ\Delta. Furthermore, if Δ\Delta is the boundary complex of a simplicial dd-polytope, then the exceptional dd-cell is also the boundary complex of a simplicial dd-polytope.

The conjecture is proposed as a possible characterization of all (d1)(d-1)-spheres with gk=1g_k=1, without assumptions on the dimensions of missing faces, and is motivated by the generalized lower bound theorem. Its resolution status is not specified in the source.

Sources & referencesView supporting material

Primary source

Isabella Novik and Hailun Zheng, “Simplicial spheres with g_k=1”, arXiv:2601.10072 (2026).

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