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A006239
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Row 3 of array in A212801.
(Formerly M4909)
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2
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1, 13, 108, 793, 5611, 39312, 274933, 1923025, 13455396, 94169413, 659134543, 4613813568, 32296413241, 226074381637, 1582520088348, 11077641280225, 77543496352291, 542804506787088, 3799631657379853, 26597421924762793
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,2
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COMMENTS
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Number of Eulerian circuits in the Cartesian product of two directed cycles of lengths 3 and n. - Andrew Howroyd, Jan 14 2018
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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Eric Weisstein's World of Mathematics, Checkers.
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FORMULA
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Empirical g.f.: x*(1-7*x^2)/((1-x)*(1-7*x)*(1-5*x+7*x^2)). - Bruno Berselli, May 31 2012
Empirical closed form: a(n) = (2^n*(1+7^n) -(5-i*sqrt(3))^n -(5+i*sqrt(3))^n) / (3*2^n), where i=sqrt(-1). - Bruno Berselli, May 31 2012
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MATHEMATICA
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T[m_, n_] := Product[2 - Exp[2*I*h*Pi/m] - Exp[2*I*k*Pi/n], {h, 1, m - 1}, {k, 1, n - 1}];
a[n_] := T[3, n] // Round;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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