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Search: id:A000985
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| A000985 |
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Number of n X n symmetric matrices with nonnegative entries and all row sums 2.
(Formerly M2907 N1168)
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+0
6
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| 1, 1, 3, 11, 56, 348, 2578, 22054, 213798, 2313638, 27627434, 360646314, 5107177312, 77954299144, 1275489929604, 22265845018412, 412989204564572, 8109686585668956, 168051656468233972, 3664479286118269972, 83868072451846938336, 2009964340465840802576
(list; graph; listen)
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OFFSET
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0,3
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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).
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
H. Gupta, Enumeration of symmetric matrices, Duke Math. J., 35 (1968), 653-659.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Example 5.2.7.
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LINKS
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T. D. Noe, Table of n, a(n) for n=0..100
P. Flajolet and R. Sedgewick, Analytic Combinatorics, 2009; see page 584
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FORMULA
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E.g.f.: (1-x)^(-1/2)*exp(x^2/4 + x/((2*(1-x)))).
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CROSSREFS
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Cf. A000986.
Sequence in context: A136104 A007841 A036760 this_sequence A094611 A052442 A001586
Adjacent sequences: A000982 A000983 A000984 this_sequence A000986 A000987 A000988
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KEYWORD
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nonn,nice,easy
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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