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A140860 Number of inequivalent regular SQ matrices (strongly quadrangular) of order n. 0
1, 2, 2, 4, 3, 9 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Abstract: Strongly quadrangular matrices have been introduced in the study of the combinatorial properties of unitary matrices. It is known that if a (0, 1)-matrix supports a unitary then it is strongly quadrangular. However, the converse is not necessarily true. In this paper, we fully classify strongly quadrangular matrices up to degree 5. We prove that the smallest strongly quadrangular matrices which do not support unitaries have exactly degree 5. Further, we isolate two submatrices not allowing a (0, 1)-matrix to support unitaries.

LINKS

Table of n, a(n) for n=1..6.

Simone Severini and Ferenc Szollosi, A further look into combinatorial orthogonality

CROSSREFS

Sequence in context: A005176 A050335 A223537 * A019681 A241580 A323257

Adjacent sequences: A140857 A140858 A140859 * A140861 A140862 A140863

KEYWORD

nonn

AUTHOR

Jonathan Vos Post, Jul 18 2008

STATUS

approved

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Last modified January 31 01:17 EST 2023. Contains 359947 sequences. (Running on oeis4.)