Path counting and rank gaps in differential posets
We study the gaps $Δp_n$ between consecutive rank sizes in $r$-differential posets by introducing a projection operator whose matrix entries can be expressed in terms of the number of certain paths in the Hasse diagram. We strengthen Miller's result that $Δp_n \geq 1$, which resolved a longstanding conjecture of Stanley, by showing that $Δp_n \geq 2r$. We also obtain stronger bounds in the case that the poset has many substructures called threads.
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