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A055549 Number of normal n X n (-1,0,1)-matrices. 3
3, 33, 939, 75041, 15901363, 10833591489 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
REFERENCES
W. H. Press et al., Numerical Recipes, Cambridge, 1986; Chapter 11.
LINKS
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
Sequence in context: A234715 A126466 A002112 * A086894 A255930 A255883
KEYWORD
nonn,more,hard
AUTHOR
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

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)