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Disjoint critical sets in latin squares

ABSTRACT: A critical set in a Latin square is a subset of the Latin square containing just enough information to determine the complete Latin square. It has been conjectured that the smallest possible critical set in a Latin square is of size b n

Disjoint critical sets in Latin squares Peter Adams, Richard Bean and Abdollah Khodkar Centre for Discrete Mathematics and Computing Department of Mathematics The University of Queensland Queensland 4072 Australia

ABSTRACT: A critical set in a Latin square is a subset of the Latin square containing just enough information to determine the complete Latin square. It has been conjectured that the smallest possible critical set in a Latin square is 2 of size b n c. If this conjecture is true, it may be possible to partition a Latin 4 square L into two, three or four disjoint critical sets in L. We give a theorem to show that for a given order n, there exists a back-circulant Latin square of order n which may be partitioned into four disjoint critical sets, and we give examples of all possible di erent partitions of Latin squares of order at most 6. We also give an example of two mutually complementary critical sets, which partition a Latin square of order 8 into two disjoint critical sets.

1 Introduction A partial Latin square P of order n is an n n array containing symbols chosen from a set N of size n in such a way that each element of N occurs at most once in each row and at most once in each column of the array. For ease of exposition, a partial Latin square P will be represented by a set of ordered triples f(i; j; k) j element k 2 N occurs in cell (i; j ) of the arrayg. If all the cells of the array are lled then the partial Latin square is termed a Latin square. That is, a Latin square L of order n is an n n array with entries chosen from the set N in such a way that each element of N occurs precisely once in each row and precisely once in each column of the array. A critical set in a Latin square L (of order n) is a partial Latin square C in L, such that (1) L is the only Latin square of order n which has element k in cell (i; j ) for each (i; j; k) 2 C, and (2) no proper subset of C satis es (1). 1

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