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A179043
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Number of n X n checkered tori.
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2
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2, 7, 64, 4156, 1342208, 1908897152, 11488774559744, 288230376353050816, 29850020237398264483840, 12676506002282327791964489728, 21970710674130840874443091905462272, 154866286100907105149651981766316633972736
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OFFSET
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1,1
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COMMENTS
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Consider an n X n checkerboard whose tiles are assigned colors 0 and 1, at random. There are 2^(n^2) such checkerboards. We identify the opposite edges of each checkerboard, thus making it into a (topological) torus. There are a(n) such (distinct) tori. It is possible to show that a(n) >= 2^(n^2)/n^2 for all n.
Main diagonal of A184271.
Main diagonal of Table 3: The number a(m, n) of toroidal m x n binary arrays, allowing rotation of the rows and/or the columns but not reflection, for m, n = 1, 2, . . . , 8, at page 5 of Ethier. [Jonathan Vos Post, Jan 14, 2013]
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LINKS
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Table of n, a(n) for n=1..12.
S. N. Ethier, Counting toroidal binary arrays, arXiv:1301.2352v1 [math.CO], Jan 10, 2013.
Wikipedia, Pólya enumeration theorem
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FORMULA
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a(n) = (1/n^2)*Sum_{ c divides n } Sum_{ d divides n } phi(c)*phi(d)*2^(n^2/lcm(c,d)), where phi is A000010 and lcm stands for least common multiple. - Stewart N. Ethier, Aug 24 2012
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MATHEMATICA
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a[n_] := Sum[If[Mod[n, c] == 0, Sum[If[Mod[n, d] == 0, EulerPhi[c] EulerPhi[d] 2^(n^2/LCM[c, d]), 0], {d, 1, n}], 0], {c, 1, n}]/n ^2
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CROSSREFS
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Cf. A184271 (n X k toroidal binary arrays).
Sequence in context: A011821 A117263 A046855 * A116985 A042051 A196925
Adjacent sequences: A179040 A179041 A179042 * A179044 A179045 A179046
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KEYWORD
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nonn
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AUTHOR
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Rouben Rostamian (rostamian(AT)umbc.edu), Jun 25 2010
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EXTENSIONS
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Terms a(6) and a(7) from A184271.
a(8)-a(12) from Stewart N. Ethier, Aug 24 2012
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STATUS
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approved
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