Maximising the permanent of (0,1) matrices and the number of extensions of Latin rectangles

Let k≥2, m≥5 and n=mk be integers. By finding bounds for certain rook polynomials, we identify the k×n Latin rectangles with the most extensions to (k+1)×n Latin rectangles. Equivalently, we find the (n-k)-regular subgraphs of Kn,n which have the greatest number of perfect matchings, and the (0,1)-matrices with exactly k zeroes in every row and column which maximise the permanent. Without the restriction on n being a multiple of k we solve the above problem (and the corresponding minimisation problem) for k=2. We also provide some computational results for small values of n and k.

Our results partially settle two open problems of Minc and conjectures by Merriell, and Godsil and McKay.