A055549 Number of normal n X n (-1,0,1)-matrices.

3, 33, 939, 75041, 15901363, 10833591489

Offset

1,1

References

W. H. Press et al., Numerical Recipes, Cambridge, 1986; Chapter 11.

Links

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

Georg Muntingh, Sage code for recursively computing higher entries

Eric Weisstein's World of Mathematics, Normal Matrix.

Mathematica

normalQ[a_] := Block[{b=Transpose[a]}, a.b === b.a]; cnt[n_]:= Block[{c=0, q=n^2}, Do[If[normalQ@Partition[IntegerDigits[t-1, 3, q]-1, n], c++ ], {t, 3^q}]; c]; Array[cnt, 4]; - Giovanni Resta, Apr 03 2006

Crossrefs

Cf. A055547, A055548.

Sequence in context: A234715 A126466 A002112 * A086894 A255930 A255883

Adjacent sequences: A055546 A055547 A055548 * A055550 A055551 A055552

Keyword

nonn,more,hard

Author

Eric W. Weisstein

Extensions

a(4) from Giovanni Resta, Apr 03 2006

a(5) from Georg Muntingh, Feb 03 2014

a(6) from Bert Dobbelaere, Sep 20 2020

Status

approved