Latin squares and their defining sets

Abstract

A Latin square $L(n,k)$ is a square of order $n$ with its entries colored with $k$ colors so that all the entries in a row or column have different colors. Let $d(L(n,k))$ be the minimal number of colored entries of an $n\times n$ square such that there is a unique way of coloring of the yet uncolored entries in order to obtain a Latin square $L(n,k)$. In this paper we discuss the properties of $d(L(n,k))$ for $k=2n-1$ and $k=2n-2$. We give an alternate proof of the identity $d(L(n,2n-1))=n^2-n$, which holds for even $n$, and we establish the new result $d(L(n,2n-2))\ge n^2-\lfloor \frac{8n}{5}\rfloor$ and show that this bound is tight for $n$ divisible by 10.

16 pages, 24 figures