OFFSET
1,3
EXAMPLE
For n = 2 the only example is the identity matrix.
MATHEMATICA
triamat[li_List] := Block[{len=Sqrt[8Length[li]+1]/2-1/2}, If[IntegerQ[len], Part[li, # ]& /@ Table[If[j>i, j(j-1)/2+i, i(i-1)/2+j], {i, len}, {j, len}], li]]; Table[it=triamat/@ IntegerDigits[Range[0, -1+2^(n(n+1)/2)], 2, n(n+1)/2]; Count[it, (q_)?MatrixQ/; (Det[q]>0)], {n, 5}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Wouter Meeussen Aug 23, 2003.
EXTENSIONS
a(6) and a(7) from Giovanni Resta, May 08 2006
STATUS
approved