Definition: A poset $P$ is called a binomial poset if it satisfy a. $P$ is locally finite with a $\hat{0}$, and contains a infinite chain. b. Every interval $[x, y]$ of $P$ is graded. If $l(x,y)$ = n, then we call $[x,y]$ an n-interval. c. For all $n \in \mathbb{N}$, any two $n$-intervals contain the same number of maximal chains.
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To avoid creating new binomial posets by taking two of them with the same factorial function (i.e., number of maximal chains in an $n$-interval) and identifying their least elements, we should add the extra condition that there exists a maximal chain $\hat{0}< x _0< x _1< \cdots$ such that every element $x$ satisfies $x\leq x_n$ for some $n$. Backelin has constructed an uncountable number of nonisomorphic such binomial posets, all with the same factorial function. Most of these have the property that they contain two nonisomorphic intervals of the same length. See http://front.math.ucdavis.edu/0508.5397. |
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