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.

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

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

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

3. ${\displaystyle x\in P}$ and ${\displaystyle y\in Q}$.

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

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

Then ${\displaystyle P*Q}$ is the poset with the Hasse diagram below.

The star product of Eulerian posets is Eulerian.

• Product order, a different way of combining posets

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

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