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 A080636 Number of real invertible (0,1) n X n matrices with (-1,0,1) inverses. 0
 1, 4, 28, 334, 7906 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS 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 A193198 Adjacent sequences:  A080633 A080634 A080635 * A080637 A080638 A080639 KEYWORD hard,nonn AUTHOR Wouter Meeussen, Aug 25 2003 STATUS approved

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Last modified October 22 18:55 EDT 2018. Contains 316500 sequences. (Running on oeis4.)