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

3, 33, 939, 75041, 15901363, 10833591489

Offset: 1

Author

Eric W. Weisstein

Keywords

• nonn
• more
• hard

References

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

Links

• Georg Muntingh, Sage code for recursively computing higher entries
• Eric Weisstein's World of Mathematics, Normal Matrix.

Crossrefs

Cf. A055547, A055548.

Programs

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 *)

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