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The Encyclopedia of Integer Sequences
"... This article gives a brief introduction to the OnLine Encyclopedia of Integer Sequences (or OEIS). The OEIS is a database of nearly 90,000 sequences of integers, arranged lexicographically. The entry for a sequence lists the initial terms (50 to 100, if available), a description, formulae, programs ..."
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Cited by 630 (15 self)
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This article gives a brief introduction to the OnLine Encyclopedia of Integer Sequences (or OEIS). The OEIS is a database of nearly 90,000 sequences of integers, arranged lexicographically. The entry for a sequence lists the initial terms (50 to 100, if available), a description, formulae, programs to generate the sequence, references, links to relevant web pages, and other
Hadamard Matrices, Orthogonal Designs and Construction Algorithms
"... We discuss algorithms for the construction of Hadamard matrices. We include discussion of construction using Williamson matrices, Legendre pairs and the discret Fourier transform and the two circulants construction. Next we move to algorithms to determine the equivalence of Hadamard matrices using t ..."
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Cited by 4 (1 self)
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We discuss algorithms for the construction of Hadamard matrices. We include discussion of construction using Williamson matrices, Legendre pairs and the discret Fourier transform and the two circulants construction. Next we move to algorithms to determine the equivalence of Hadamard matrices using the pro le and projections of Hadamard matrices. A summary is then given which considers inequivalence of Hadamard matrices of orders up to 44. The nal two sections give algorithms for constructing orthogonal designs, short amicable and amicable sets for use in the Kharaghani array. 1 Algorithms for constructing Hadamard matrices 1.1 Hadamard matrices constructed from Williamson matrices An Hadamard matrix H of order n has elements 1 and satis es HH T = nI n . These matrices are used extensively in coding and communications (see Seberry and Yamada [90]). The order of an Hadamard matrix is 1, 2 or n (0 mod 4). The rst unsolved case is order 428. We use Williamson's construction as the basis of our algorithm to construct a distributed computer search for new Hadamard matrices. We briey describe the theory of Williamson's construction below. Previous computer searches for Hadamard matrices using Williamson's condition 2 are described in Section 1.1.1. The implementation of the search algorithm is presented in Section 1.1.2, and the results of the search are described in Section 1.1.3. Theorem 1 (Williamson [104]) Suppose there exist four (1; 1) matrices A, B, C, D of order n which satisfy XY T = Y X T ; X;Y 2 fA; B; C; Dg Further, suppose AA T +BB T + CC T +DD T = 4nI n (1) Then H = 2 6 6 6 4 A B C D B A D C C D A B D C B A 3 7 7 7 5 (2) is an Hadamard matrix of order 4n constructed from a Williamson array. Let the matrix T given below be called ...
Largedeterminant sign matrices of order 4k + 1
 Discrete Math
"... Abstract. The Hadamard maximal determinant problem asks for the largest n × n determinant with entries ±1. When n ≡ 1 (mod 4) , the maximal excess construction of Farmakis & Kounias [FK] has been the most successful general method for constructing large (though seldom maximal) determinants. For cert ..."
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Abstract. The Hadamard maximal determinant problem asks for the largest n × n determinant with entries ±1. When n ≡ 1 (mod 4) , the maximal excess construction of Farmakis & Kounias [FK] has been the most successful general method for constructing large (though seldom maximal) determinants. For certain small n, however, still larger determinants have been known; several new records were recently reported in [OSDS]. Here, we define “3normalized ” n × n Hadamard matrices, and construct largedeterminant matrices of order n + 1 from them. Our constructions account for most of the previous “small n ” records,
SWITCHING OPERATIONS FOR HADAMARD MATRICES
, 2005
"... Abstract. We define several operations that switch substructures of Hadamard matrices thereby producing new, generally inequivalent, Hadamard matrices. These operations have application to the enumeration and classification of Hadamard matrices. To illustrate their power, we use them to greatly impr ..."
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Abstract. We define several operations that switch substructures of Hadamard matrices thereby producing new, generally inequivalent, Hadamard matrices. These operations have application to the enumeration and classification of Hadamard matrices. To illustrate their power, we use them to greatly improve the lower bounds on the number of equivalence classes of Hadamard matrices in orders 32 and 36 to 3,578,006 and 4,745,357. 1.
On the classification of Hadamard matrices of order 32
, 2009
"... All equivalence classes of Hadamard matrices of order at most 28 have been found by 1994. Order 32 is where a combinatorial explosion occurs on the number of inequivalent Hadamard matrices. We find all equivalence classes of Hadamard matrices of order 32 which are of certain types. It turns out that ..."
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Cited by 2 (0 self)
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All equivalence classes of Hadamard matrices of order at most 28 have been found by 1994. Order 32 is where a combinatorial explosion occurs on the number of inequivalent Hadamard matrices. We find all equivalence classes of Hadamard matrices of order 32 which are of certain types. It turns out that there are exactly 13,680,757 Hadamard matrices of one type and 26,369 such matrices of another type. Based on experience with the classification of Hadamard matrices of smaller order, it is expected that the number of the remaining two types of these matrices, relative to the total number of Hadamard matrices of order 32, to be insignificant.
A SENSITIVE ALGORITHM FOR DETECTING THE INEQUIVALENCE OF HADAMARD MATRICES
"... Abstract. A Hadamard matrix of side n is an n × n matrix with every entry either 1 or −1, which satisfies HH T = nI. Two Hadamard matrices are called equivalent if one can be obtained from the other by some sequence of row and column permutations and negations. To identify the equivalence of two Had ..."
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Abstract. A Hadamard matrix of side n is an n × n matrix with every entry either 1 or −1, which satisfies HH T = nI. Two Hadamard matrices are called equivalent if one can be obtained from the other by some sequence of row and column permutations and negations. To identify the equivalence of two Hadamard matrices by a complete search is known to be an NP hard problem when n increases. In this paper, a new algorithm for detecting inequivalence of two Hadamard matrices is proposed, which is more sensitive than those known in the literature and which has a close relation with several measures of uniformity. As an application, we apply the new algorithm to verify the inequivalence of the known 60 inequivalent Hadamard matrices of order 24; furthermore, we show that there are at least 382 pairwise inequivalent Hadamard matrices of order 36. The latter is a new discovery. 1.
On Circulant Best Matrices and Their Applications
"... Call four type 1 (1; \Gamma1) matrices, X 1 ; X 2 ; X 3 ; X 4 , of the same group of order m (odd) with the properties (i) (X i \GammaI ) T = \Gamma(X i \GammaI ); i = 1; 2; 3 ; (ii) X T 4 = X 4 and the diagonal elements are positive, (iii) X i X j = X j X i and (iv) X 1 X T 1 +X 2 X T 2 +X 3 X T 3 ..."
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Call four type 1 (1; \Gamma1) matrices, X 1 ; X 2 ; X 3 ; X 4 , of the same group of order m (odd) with the properties (i) (X i \GammaI ) T = \Gamma(X i \GammaI ); i = 1; 2; 3 ; (ii) X T 4 = X 4 and the diagonal elements are positive, (iii) X i X j = X j X i and (iv) X 1 X T 1 +X 2 X T 2 +X 3 X T 3 +X 4 X T 4 = 4mIm ; best matrices. We use a computer to give, for the first time, all inequivalent best matrices of odd order m 31. Inequivalent best matrices of order m, m odd, can be used to find inequivalent skewHadamard matrices of order 4m. We use best matrices of order 1 4 (s 2 +3) to construct new orthogonal designs, including new OD(2s 2 +6; 1; 1; 2; 2; s 2 ; s 2 ). AMS Subject Classification: Primary 05B20, Secondary 05B30 Key words and phrases: Circulant matrices, supplementary difference sets, orthogonal designs, Hadamard matrices. 1