Zig-zag product

The zig-zag product is an operation on graphs, specifically useful in the field of combinatorial design and expander graphs. It allows the construction of a new graph from two existing graphs in a way that preserves certain properties, typically expanding size and connectivity characteristics. For two graphs \( G \) and \( H \): - Let \( G \) be a graph with vertex set \( V_G \) and \( H \) be a directed graph with vertex set \( V_H \).