

A007299


Number of Hadamard matrices of order 4n.
(Formerly M3736)


24




OFFSET

0,5


COMMENTS

More precisely, number of inequivalent Hadamard matrices of order n if two matrices are considered equivalent if one can be obtained from the other by permuting rows, permuting columns and multiplying rows or columns by 1.
The Hadamard conjecture is that a(n) > 0 for all n >= 0.  Charles R Greathouse IV, Oct 08 2012
From Bernard Schott, Apr 24 2022: (Start)
A brief historical overview based on the article "La conjecture de Hadamard" (see link):
1893  J. Hadamard proposes his conjecture: a Hadamard matrix of order 4k exists for every positive integer k (see link).
As of 2000, there were five multiples of 4 less than or equal to 1000 for which no Hadamard matrix of that order was known: 428, 668, 716, 764 and 892.
2005  Hadi Kharaghani and Behruz TayfehRezaie publish their construction of a Hadamard matrix of order 428 (see link).
2007  D. Z. Djoković publishes "Hadamard matrices of order 764 exist" and constructs 2 such matrices (see link).
As of today, there remain 13 multiples of 4 less than or equal to 2000 for which no Hadamard matrix of that order is known: 668, 716, 892, 1004, 1132, 1244, 1388, 1436, 1676, 1772, 1916, 1948, and 1964. (End)


REFERENCES

J. Hadamard, Résolution d'une question relative aux déterminants, Bull. des Sciences Math. 2 (1893), 240246.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
Wolfram, S. A New Kind of Science. Champaign, IL: Wolfram Media, p. 1073, 2002.


LINKS

Table of n, a(n) for n=0..8.
V. Alvarez, J. A. Armario, M. D. Frau and F. Gudlel, The maximal determinant of cocyclic (1, 1)matrices over D_{2t}, Linear Algebra and its Applications, 2011.
F. J. Aragon Artacho, J. M. Borwein, and M. K. Tam, DouglasRachford Feasibility Methods for Matrix Completion Problems, March 2014.
Dragomir Z. Djoković, Hadamard matrices of order 764 exist, arXiv:math/0703312 [math.CO], 2007.
Shalom Eliahou, La conjecture de Hadamard (I), Images des Mathématiques, CNRS, 2020.
Jacques Salomon Hadamard, "Sur le module maximum que puisse atteindre un déterminant", C. R. Acad. Sci. Paris 116 (1893) 15001501 (link from Gallica).
Hadi Kharaghani, Home Page
Hadi Kharaghani and B. TayfehRezaie, Hadamard matrices of order 32, J. Combin. Designs 21 (2013) no. 5, 212221. [DOI]
Hadi Kharaghani and B. TayfehRezaie, A Hadamard matrix of order 428, Journal of Combinatorial Designs, Volume 13, Issue 6, November 2005, pp. 435440 (First published: 13 December 2004).
H. Kharaghani and B. TayfehRezaie, On the classification of Hadamard matrices of order 32, J. Combin. Des., 18 (2010), 328336.
H. Kharaghani and B. TayfehRezaie, Hadamard matrices of order 32, 2012.
H. Kimura, Hadamard matrices of order 28 with automorphism groups of order two, J. Combin. Theory (1986), A 43, 98102.
H. Kimura, New Hadamard matrix of order 24, Graphs Combin. (1989), 5, 235242.
H. Kimura, Classification of Hadamard matrices of order 28 with Hall sets, Discrete Math. (1994), 128, 257268.
H. Kimura, Classification of Hadamard matrices of order 28, Discrete Math. (1994), 133, 171180.
W. P. Orrick, Switching operations for Hadamard matrices, arXiv:math/0507515 [math.CO], 20052007. (Gives lower bounds for a(8) and a(9))
N. J. A. Sloane, Tables of Hadamard matrices
N. J. A. Sloane, My favorite integer sequences, in Sequences and their Applications (Proceedings of SETA '98).
Warren D. Smith, C program for generating Hadamard matrices of various orders
Edward Spence, Classification of Hadamard matrices of order 24 and 28, Discrete Math. 140 (1995), no. 13, 185243.
Eric Weisstein's World of Mathematics, Hadamard Matrix
Index entries for sequences related to Hadamard matrices


CROSSREFS

Cf. A019442, A096201, A036297, A048615, A048616, A003432, A048885.
Sequence in context: A343290 A027858 A181755 * A257935 A109254 A258091
Adjacent sequences: A007296 A007297 A007298 * A007300 A007301 A007302


KEYWORD

hard,nonn,nice


AUTHOR

N. J. A. Sloane


EXTENSIONS

a(8) from the H. Kharaghani and B. TayfehRezaie paper.  N. J. A. Sloane, Feb 11 2012


STATUS

approved



