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A055549 Number of normal n X n (-1,0,1)-matrices. 3
3, 33, 939, 75041, 15901363 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

REFERENCES

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

LINKS

Table of n, a(n) for n=1..5.

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

STATUS

approved

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Last modified October 22 12:22 EDT 2018. Contains 316446 sequences. (Running on oeis4.)