%I #19 Sep 20 2020 03:24:44
%S 3,33,939,75041,15901363,10833591489
%N Number of normal n X n (-1,0,1)-matrices.
%D W. H. Press et al., Numerical Recipes, Cambridge, 1986; Chapter 11.
%H Georg Muntingh, <a href="/A055549/a055549.txt">Sage code for recursively computing higher entries</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/NormalMatrix.html">Normal Matrix.</a>
%t 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
%Y Cf. A055547, A055548.
%K nonn,more,hard
%O 1,1
%A _Eric W. Weisstein_
%E a(4) from _Giovanni Resta_, Apr 03 2006
%E a(5) from _Georg Muntingh_, Feb 03 2014
%E a(6) from _Bert Dobbelaere_, Sep 20 2020