Incomplete conjugate orthogonal idempotent latin squares

Large sets of resolvable idempotent Latin squares

A Latin square has six conjugate Latin squares obtained by uniformly permuting its (row, column, symbol) triples. We say that a Latin square has conjugate symmetry if at least two of its six conjugates are equal. We enumerate Latin squares with conjugate symmetry and classify them according to several common notions of equivalence. We also do similar enumerations under additional hypotheses, such as assuming the Latin square is reduced, diagonal, idempotent or unipotent. Our data corrected an error in earlier literature and suggested several patterns that we then found proofs for, including (1) the number of isomorphism classes of semisymmetric idempotent Latin squares of order equals the number of isomorphism classes of semisymmetric unipotent Latin squares of order , and (2) suppose and are totally symmetric Latin squares of order . If and are paratopic then and are isomorphic.

Latin squares have wide application in communication and cipher. Denote by IdFin(v) the set of all integer pairs (t, s) for which there exist three idempotent Latin squares of order r on the same set having fine structure (t, s). We obtain that (6, 17), (4, 18)∈IdFin(7), which gives some new examples (6, 17), (4, 18)∈Fin(7). We prove that (t, 16)∉IdFin(v) with 0t3 for order v with 8  v  11, and determine the set IdFin(11).

The paper is devoted to discovering new features of diagonal Latin squares of small order. We present an algorithm, based on a special kind of transformations, that constructs a canonical form of a given diagonal Latin square. Each canonical form corresponds to one isotopy class of diagonal Latin squares. The algorithm was implemented and used to enumerate the isotopy classes of diagonal Latin squares of order at most 8. For order 8 the computational experiment was conducted in a volunteer computing project. The algorithm was also used to estimate how long it would take to enumerate the isotopy classes of diagonal Latin squares of order 9 in the same volunteer computing project.

Embedding partial Latin squares in Latin squares with many mutually orthogonal mates

Latin squares have wide application in communication and cipher. Denote by IdFin(r) the set of all integer pairs (t, s) for which there exist three idempotent Latin squares of order v on the same set having fine structure (t, s). A method to construct elements of IdFin(2v+1) by using elements in IdFin(v) is presented.

Suppose that L is a latin square of order m and P ⊑ L is a partial latin square. If L is the only latin square of order m which contains P, and no proper subset of P has this property, then P is a critical set of L. The critical set spectrum problem is to determine, for a given m, the set of integers t for which there exists a latin square of order m with a critical set of size t. We outline a partial solution to the critical set spectrum problem for latin squares of order 2n. The back circulant latin square of even order m has a well‐known critical set of size m2/4, and this is the smallest known critical set for a latin square of order m. The abelian 2‐group of order 2n has a critical set of size 4n‐3n, and this is the largest known critical set for a latin square of order 2n. We construct a set of latin squares with associated critical sets which are intermediate between the back circulant latin square of order 2n and the abelian 2‐group of order 2n. © 2007 Wiley Periodicals, Inc. J Combin Designs 16: 25–43, 2008

Indivisible partitions of latin squares

The maximum number of mutually orthogonal Sudoku Latin squares (MOSLS) of order $$n=m^2$$n=m2 is $$n-m$$n-m. In this paper, we construct for $$n=q^2$$n=q2, q a prime power, a set of $$q^2-q-1$$q2-q-1 MOSLS of order $$q^2$$q2 that cannot be extended to a set of $$q^2-q$$q2-q MOSLS. This contrasts to the theory of ordinary Latin squares of order n, where each set of $$n-2$$n-2 mutually orthogonal Latin Squares (MOLS) can be extended to a set of $$n-1$$n-1 MOLS (which is best possible). For this proof, we construct a particular maximal partial spread of size $$q^2-q+1$$q2-q+1 in $$\mathrm {PG}(3,q)$$PG(3,q) and use a connection between Sudoku Latin squares and projective geometry, established by Bailey, Cameron and Connelly.

In a latin square of order n, a k-plex is a selection of kn entries in which each row, column, and symbol occurs k times. A 1-plex is also called a transversal. A k-plex is indivisible if it contains no c-plex for 0<c<k. We prove that, for all n≥4, there exists a latin square of order nthat can be partitioned into an indivisible ⌊n/2⌋-plex and a disjoint indivisible ⌈n/2⌉-plex. For all n≥3, we prove that there exists a latin square of order n with two disjoint indivisible ⌊n/2⌋-plexes. We also give a short new proof that, for all odd n≥5, there exists a latin square of order n with at least one entry not in any transversal. Such latin squares have no orthogonal mate. Copyright © 2011 Wiley Periodicals, Inc. J Combin Designs 19:304-312, 2011

Diagonally cyclic latin squares

An orthogonal latin square graph (OLSG) is one in which the vertices are latin squares of the same order and on the same symbols, and two vertices are adjacent if and only if the latin squares are orthogonal. If G is an arbitrary finite graph, we say that G is realizable as an OLSG if there is an OLSG isomorphic to G. The spectrum of G [Spec(G)] is defined as the set of all integers n that there is a realization of G by latin squares of order n. The two basic theorems proved here are (1) every graph is realizable and (2) for any graph G, Spec G contains all but a finite set of integers. A number of examples are given that point to a number of wide open questions. An example of such a question is how to classify the graphs for which a given n lies in the spectrum.

ABSTRACTA Latin square of order is an matrix in which each row and column contains each of symbols exactly once. For , we show that with high probability a uniformly random Latin square of order has no proper subsquare of order larger than . Using this fact, we present a canonical labeling algorithm for Latin squares of order that runs in average time bounded by a polynomial in . The algorithm can be used to solve isomorphism problems for many combinatorial objects that can be encoded using Latin squares, including quasigroups, Steiner triple systems, Mendelsohn triple systems, 1‐factorizations, nets, affine planes, and projective planes.

In this paper we consider the approach to search for systems of orthogonal Latin Squares, that is based on reducing the corresponding problems to Boolean Satisfiability problem. We constructed the SAT encoding for finding orthogonal diagonal Latin Squares of order 10. Using this encoding and the resources provided by the volunteer computing project SAT@home we managed to find 17 previously unknown pairs. Based on these 17 pairs and on 3 previously published pairs we constructed the pseudotriples of diagonal Latin Squares of order 10. To construct pseudotriples we employed the computing cluster. The last step required us to make parallel implementation of the algorithm for generating diagonal Latin Squares of order 10.

Suppose that and . We construct a Latin square of order n with the following properties: has no proper subsquares of order 3 or more. has exactly one intercalate (subsquare of order 2). When the intercalate is replaced by the other possible subsquare on the same symbols, the resulting Latin square is in the same species as . Hence generalizes the square that Sade famously found to complete Norton's enumeration of Latin squares of order 7. In particular, is what is known as a self-switching Latin square and possesses a near-autoparatopism.