An improved lower bound for maximal partial Room squares

Abstract

We establish a new asymptotic lower bound for the volume of a maximal partial Room square of even order $n$. We prove that its volume is at least $\frac{2\sqrt{66}-6}{57}n^2-O(n)$, improving the leading coefficient $1/8$ in the lower bound obtained by Meszka and Rosa. The proof combines two counting arguments, one based on a transversal of empty cells and the other on the graph of occurring symbol pairs.