Star product

Template:Distinguish In mathematics, the star product is a method of combining graded posets with unique minimal and maximal elements, preserving the property that the posets are Eulerian.

Definition

The star product of two graded posets $(P,{\le }_P)$ and $(Q,{\le }_Q)$, where $P$ has a unique maximal element $\hat{1}$ and $Q$ has a unique minimal element $\hat{0}$, is a poset $P*Q$ on the set $(P\setminus \{\hat{1}\})\cup (Q\setminus \{\hat{0}\})$. We define the partial order ${\le }_{P*Q}$ by $x\le y$ if and only if:

1. $\{x,y\}\subset P$, and $x{\le }_{P}y$;

2. $\{x,y\}\subset Q$, and $x{\le }_{Q}y$; or

3. $x\in P$ and $y\in Q$.

In other words, we pluck out the top of $P$ and the bottom of $Q$, and require that everything in $P$ be smaller than everything in $Q$.

Example

For example, suppose $P$ and $Q$ are the Boolean algebra on two elements.

Then $P*Q$ is the poset with the Hasse diagram below.

Properties

The star product of Eulerian posets is Eulerian.

See also

• Product order, a different way of combining posets

References

• Stanley, R., Flag $f$-vectors and the $𝐜𝐝$-index, Math. Z. 216 (1994), 483-499.