A $\textit{sigma partitioning}$ of a graph $G$ is a partition of the vertices into sets $P_1. \ldots. P_k$ such that for every two adjacent vertices $u$ and $v$ there is an index $i$ such that $u$ and $v$ have different numbers of neighbors in $P_i$. The $\textit{ sigma number}$ of a graph $G$. https://chefesquipmenters.shop/product-category/waffle-pans/