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A135588
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Number of symmetric (0,1)-matrices with exactly n entries equal to 1 and no zero rows or columns.
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3
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1, 1, 2, 6, 20, 74, 302, 1314, 6122, 29982, 154718, 831986, 4667070, 27118610, 163264862, 1013640242, 6488705638, 42687497378, 288492113950, 1998190669298, 14177192483742, 102856494496050, 762657487965086, 5771613810502002
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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FORMULA
| G.f.: Sum((1+x)^n*(1+x^2)^binomial(n,2)/2^(n+1),n=0..infinity).
G.f.: Sum(Sum((-1)^(n-k)*binomial(n,k)*(1+x)^k*(1+x^2)^binomial(k,2),k=0..n),n=0..infinity).
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CROSSREFS
| Cf. A104602.
Row sums of A135589.
Sequence in context: A188144 A150158 A034010 * A150159 A150160 A150161
Adjacent sequences: A135585 A135586 A135587 * A135589 A135590 A135591
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KEYWORD
| nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Feb 25 2008, Mar 03 2008, Mar 04 2008
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