A080636 - OEIS

A080636

Number of real invertible (0,1) n X n matrices with (-1,0,1) inverses.

1, 4, 28, 334, 7906

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

{{1,1,1,1,0},{1,1,1,0,1},{1,1,1,1,1},{1,0,1,1,0},{0,1,1,0,0}} qualifies since its inverse is

{{1,1,-1,0,-1},{1,0,0,-1,0},{-1,0,0,1,1},{0,-1,1,0,0},{-1,0,1,0,0}}

MATHEMATICA

triamat[li_List] := (*see A086900*); Table[it=triamat/@IntegerDigits[Range[0, -1+2^(n(n+1)/2)], 2, n(n+1)/2]; Count[it, (q_?MatrixQ)/; Det[q]=!=0 && Union[Flatten[{Inverse[q], {-1, 0, 1}}]]==={-1, 0, 1}], {n, 5}]

CROSSREFS

Sequence in context: A198513 A113371 A280570 * A223887 A140425 A377551

Adjacent sequences: A080633 A080634 A080635 * A080637 A080638 A080639

KEYWORD

hard,nonn

AUTHOR

Wouter Meeussen, Aug 25 2003

STATUS

approved