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An Algorithm for Computing the Automorphism Group of a Hadamard Matrix

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An Algorithm for Computing the Automorphism Group of a Hadamard Matrix CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector JOURNAL OF COMBINATORIAL THEORY, Series A 27, 289-306 (1979) An Algorithm for Computing the Automorphism Group of a Hadamard Matrix JEFFREY S. LEON* Department of Mathematics, University of Illinoisat Chicago Circle, Chicago, Illinois 60608 Communicated by Marshall Hall, Jr. Received October 3, 1977 An algorithm for computing the automorphism group of a Hadamard matrix is described. It is shown how the algorithm may be modified to determine if two Hadamard matrices are equivalent. A Hadamard matrix of dimension n is an n x n matrix H of - l’s and l’s such that HHT = n1. The dimension n must be 1, 2, or a multiple of 4. Hadamard matrices of dimension n are known to exist for many, but not all, values of n which are multiples of 4 (see Ref. [I]). Two Hadamard matrices HI and Hz of the same dimension are equivalent if He may be obtained from HI by a signed permutation of the rows and columns (precise definitions in Section 2); the signed permutation of rows and columns is called an iso- morphism from HI to Hz or, when H, = HI, an automorphism of HI. Under composition, the automorphisms of HI form a group called the automorphism group. This group provides a measure of the symmetry of HI ; for example, if it is transitive on rows, any row of HI is “equivalent” to any other row. This article describes an algorithm for computing the automorphism group of a Hadamard matrix. It also shows how the algorithm may be modified to determine if two Hadamard matrices are equivalent. The algorithm has been programmed (in Fortran) and run on a computer. It has proved quite efficient for matrices of moderate dimension (up to 50); probably it can be applied, under appropriate conditions, to matrices of dimension as high as 100. The algorithm yields * Work partially supported by NSF Grant MCS76-03143. Computing services used in this research were provided by the Computer Center of the University of Illinois at Chicago Circle. Their assistance is gratefully acknowledged. 289 0097-3165/79/060289-18$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved. 290 JEFFREY S. LEON (a) the order of the automorphism group, (b) the orbits of the automorphism group on the rows and columns of the matrix, and Cc) a “canonical” set of signed permutations which generate the automorphism group; modulo sign changes, these permutations form a strong generating set (relative to a suitable base) for the automorphism group acting as a permutation group on the set of rows and columns. The concepts of base and strong generating set were developed by Sims to facilitate computation in large permutation groups. They are discussed briefly in Section 4; more details may be found in Refs. [3,4]. This article is intended to be fairly self-contained; however, a familiarity with backtrack search techniques would be helpful in understanding the algorithm. Backtrack methods in general are discussed in Ref. [2]. 1. ORDINARY AND SIGNED PERMUTATIONS This section describes the notation to be used for ordinary (unsigned) and signed permutations. The symmetric group on a finite set JJ will be denoted by S(Q). Permutations will act on the right; thus (&1)~2 = b(TITa). If G is a subgroup of S(D) and lb, , b, >..., bk} C 52, then G 61hZ...bkwill denote the stabilizer in G of bl , b2 ,..., b, . The orbit of b under G will be written as bC. The identity permu- tation will be denoted by L. A subgroup G of S(L?) is semiregular on JJ if Gb = (L\ for every b E a; in this case, 1 G / divides / Q 1. Assuming that the set Sz does not itself have a unary operation minus, let *JJ denote the set of symbols {Sb 1 6 = &I, b E Sz}. Elements of H2 may be multiplied on the left by - 1 or 1 in the natural way: S1(S,b)= (S,S,)b. We will write b for (+ I)b and -b for (- I)b. A signed permutation u on J2 is a mapping from Sz into &L? such that b10# &bzu whenever b, # b, . Such a map u can be written uniquely as a product PT with T E S(Q) and p(b) = +-I for all b E 9. Composition of signed permutations is defined by 0~0~ = (~1~1)(~272) = PT, where T = TOTS and p(b) = PI(b) pZ(bTl). The inverse of pT is pT-l, where p(b) = ,@-‘). Under composition, the signed permutations on L? form a group denoted here by z(Q). This group is a semidirect product of an elementary Abelian group &,(Q) = (,u~ 1 T = L} of order 2’“i with a group S = {pT 1 p(b) = 1 for all b} isomorphic to S(Q). The center Z(J?) of z(Q) is (&L}, where bbL = -b for all b. The map pT + T is a homomorphism from z((52) onto S(G) with kernel &,(Q). The image of a subset Q (or element 4) of z(52) under this map will be written a (or 4). Note that @ = 7. For a fixed positive integer n, let fi;2, be the set of symbols {RI, R2,..., Rn) AUTOMORPHISMS OF HADAMARD MATRICES 291 and 52, the set of symbols (Cl, C2,..., Cn>. Let Sr,, = QR u Q, . We will think of Ri and Cj as representing the ith row and jth column of an n x n matrix, respectively. Let z(QRc) denote the subgroup of zl(ln,,) mapping QR onto fiB and 1;2, onto Q,. If u E f(QRc), then (sR and uc will denote the restrictions of u to OR and 9, , respectively, and 6:n and 6’c will be the elements of z(QRC) defined by GR /RR = uR , 6.R ISac = 1, 6, /RR = l, Gc IRC = UC. Note u = ijR6.C = I?@~. Now &&?& operates on the Set of n x n (- 1, I)-matrices as follows: Tf H = (/z& u = ~7, Ri’-’ = Ri’, and Cjr-’ = Cj”, then Ho = (ai&, where aij = p(Ri’) p(Cj”) hiri- . Thus, if Ri” = SRI, &row i) of H becomes (row I) of HO, and likewise for columns. Note that (Hcl)uz = IPl”~) and H” = (H”~)“c = (HdC)“R(i.e., row and column permutations commute). Finally, we note that Z(&) is isomorphic to the group of n-dimensional (-I, I)-monomial matrices. Given u E Z(Qc), let AC(u) = (aii), aij = p(Ci) if Cr? = Cj and aij = 0 otherwise. Then 4’c(u,u,) = ~&‘c(uJ A,-(u&, and the map u --f AC(u) provides the isomorphism. Similarly, given u E z(QR), let AR(u) = (&), dii = p(Rj) if Rj’ = Ri and d,, = 0 otherwise. In this case, J~Z!~(U~UJ = dR(uS) dR(ul) and the map u + AR(u))’ provides an isomorphism from z(QR) onto the group of n-dimensional (- 1, I)-monomial matrices. Note that H3~ = .AYR(uR)Hand He= = HA%‘~(u,-). 2. AUTOMORPHISMS AND ISOMORPHISMS OF HADAMARD MATRICES An automorphism of an n-dimensional Hadamard matrix H is a signed permutation u in z(SZRC) such that H” = H. Under composition, the automorphisms of H form a subgroup of z(QRc) called the automorphism group of H and written here as AUT(H). Alternatively, the automorphism group may be defined to be the set of pairs (P, Q) of (- 1, I)-monomial matrices such that PHQ = H, composition being defined by (P, Q)(P’, Q’) = (P’P, QQ’). This definition yields an isomorphic group, the isomorphism being given by u -+ (JIR(uR), J?~(u,-)). The notation of signed permutations will be used here as we shall be interested in the permutation group properties of automorphisms. Two n-dimensional Hadamard matrices Hand H’ are equivalent if H” = H’ for some u E z(QRC). Since Ho1 = Ho2if and only if u1 and uz are in the same right coset of AUT(H) in z(QRC), the number of Hadamard matrices equivalent to His 22”(n!)2// AUT(H)I. An automorphism IJ of H is determined uniquely by either of (TR or gc . This is an immediate consequence of the equation difR(~R) HJ%‘~(uJ = H. Let AUTR(H) = {uR / u E AUT(H)) C Z(J?R) and AUT&H) = {UC 1 u E AUT(H)) C C(f&). These groups will be called the row and column auto- morphism groups of H, respectively. As abstract groups AUTR(H) and 292 JEFFREY S. LEON AUT,(H) are both isomorphic to AUT(H) (the maps u - (TR and u - uc are isomorphisms); however, they may act very differently as permutation groups; for example, AUT,(H) may be transitive on !& while AUTR(H) is intransitive on 9, . Let R,(H) = {u E AUT(H) 1OR E &(~R)) and let CO(H) = {U E AUT(H) 1 uc E &,(J2~)}. Then R,(H) and C,,(H) are normal elementary Abelian 2- subgroups of AUT(H) containing Z(H) = {&L}. In matrix notation, R,(H) corresponds to pairs (P, Q) with P diagonal and C,-,(H) to pairs (P, Q) with Q diagonal. LEMMA 2.1. Let H be an n-dimensional Hadamard matrix. (a) If u E R,,(H) and CjO = Cj for some j, then u = L. (b) R,(H) n Co(H) = Z(H). (c) AUT(H) cv AUT(H)/Z(H). (d) R,(H) N R,(H)/Z(H) restricted to 52, is semiregular; hence j R,(H)/ is a power of 2 dividing 2n.
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