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A140839 Number of inequivalent indecomposable regular SQ matrices (strongly quadrangular) of order n. 0
1, 1, 1, 2, 2, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

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: A006560 A088251 A229627 * A320299 A292943 A279906

Adjacent sequences:  A140836 A140837 A140838 * A140840 A140841 A140842

KEYWORD

nonn

AUTHOR

Jonathan Vos Post, Jul 18 2008

STATUS

approved

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Last modified January 17 10:30 EST 2019. Contains 319218 sequences. (Running on oeis4.)