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A007107
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Number of labeled 2-regular digraphs with n nodes.
(Formerly M4668)
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8
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1, 0, 0, 1, 9, 216, 7570, 357435, 22040361, 1721632024, 166261966956, 19459238879565, 2714812050902545, 445202898702992496, 84798391618743138414, 18567039007438379656471
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OFFSET
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0,5
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COMMENTS
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a(n)= sum_{k=0}^{n} sum_{s=0}^{k} sum_{j=0}^{n-k}frac{(-1)^{k+j-s}n!(n-k)!(2n-k-2j-s)!}{s!(k-s)!((n-k-j)!)^{2}j!2^{2n-2k-j}} - Shanzhen Gao, Nov 05 2007
Or number of nXn matrices with exactly two 1's in each row and column which are not in the main diagonal, other entries 0 (cf. A001499). [From Vladimir Shevelev, Mar 22 2010]
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REFERENCES
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O. Gonzalez, C. Beltran and I. Santamaria, On the Number of Interference Alignment Solutions for the K-User MIMO Channel with Constant Coefficients, arXiv preprint arXiv:1301.6196, 2013. - From N. J. A. Sloane, Feb 19 2013
R. W. Robinson, personal communication.
R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1982.
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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R. W. Robinson, Table of n, a(n) for n = 0..48
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CROSSREFS
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Sequence in context: A109587 A067426 A007108 * A217042 A064633 A084942
Adjacent sequences: A007104 A007105 A007106 * A007108 A007109 A007110
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KEYWORD
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nonn,nice
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AUTHOR
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N. J. A. Sloane.
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STATUS
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approved
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