rank unimodal poset

Let $P$ be a poset and $ρ$ a rank function on it with $l = max (ρ (P))$ (if it exists). $P$ is rank-unimodal if

r_{0} \leq r_{1} \leq \dots \leq r_{k} \geq r_{k + 1} \geq r_{l} $ $ w h e r e $ k = \frac{n}{2} $ a n d $ r_{i} $ a r e t h e [[Z e t t e l k a s t e n / M a t h / C o m b i n a t o r i c s / r a n k n u m b e r s ∥ r a n k n u m b e r s]]