Uniform semi-Latin squares and their pairwise-variance aberrations

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For integers n>2 and k>0, an (n×n)∕ksemi-Latin square is an n×n array of k-subsets (called blocks) of an nk-set (of treatments), such that each treatment occurs once in each row and once in each column of the array. A semi-Latin square is uniform if every pair of blocks, not in the same row or column, intersect in the same positive number of treatments. It is known that a uniform (n×n)∕k semi-Latin square is Schur optimal in the class of all (n×n)∕k semi-Latin squares, and here we show that when a uniform (n×n)∕k semi-Latin square exists, the Schur optimal (n×n)∕k semi-Latin squares are precisely the uniform ones. We then compare uniform semi-Latin squares using the criterion of pairwise-variance (PV) aberration, introduced by J. P. Morgan for affine resolvable designs, and determine the uniform (n×n)∕k semi-Latin squares with minimum PV aberration when there exist n−1 mutually orthogonal Latin squares of order n. These do not exist when n=6, and the smallest uniform semi-Latin squares in this case have size (6×6)∕10. We present a complete classification of the uniform (6×6)∕10 semi-Latin squares, and display the one with least PV aberration. We give a construction producing a uniform ((n+1)×(n+1))∕((n−2)n) semi-Latin square when there exist n−1 mutually orthogonal Latin squares of order n, and determine the PV aberration of such a uniform semi-Latin square. Finally, we describe how certain affine resolvable designs and balanced incomplete-block designs can be constructed from uniform semi-Latin squares. From the uniform (6×6)∕10 semi-Latin squares we classified, we obtain (up to block design isomorphism) exactly 16875 affine resolvable designs for 72 treatments in 36 blocks of size 12 and 8615 balanced incomplete-block designs for 36 treatments in 84 blocks of size 6. In particular, this shows that there are at least 16875 pairwise non-isomorphic orthogonal arrays OA(72,6,6,2).