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The $g$-conjecture (or $g$-theorem in the polytopal case) aims the characterize what possible $f$-vectors can arise from simplicial spheres. Here is some nice history and introduction to the $g$-conjecture by Gil Kalai. In brief the $g$-conjecture says that for a simplicial sphere the $h$-vector should be symmetric and the $g$-vector should be an $M$-vector where $g_i = h_i - h_{i-1}$.