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A059947
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Number of 6-block bicoverings of an n-set.
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2
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0, 0, 0, 3, 256, 7255, 149660, 2681063, 44659776, 714287535, 11154475420, 171673613023, 2618246526896, 39701554817015, 599773397512380, 9038881598035383, 136004367641775616, 2044264589908169695
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,4
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REFERENCES
| I. P. Goulden and D. M.Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
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FORMULA
| a(n)=(1/6!)*(15^n-6*10^n-15*7^n+30*6^n+60*4^n-50*3^n-180*2^n+240). E.g.f. for m-block bicoverings of an n-set is exp(-x-1/2*x^2*(exp(y)-1))*Sum_{i=0..inf} x^i/i!*exp(binomial(i, 2)*y).
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CROSSREFS
| Cf. A002718, A059443, A003462, A059945, A059946, A059948-A059951.
Sequence in context: A028918 A045824 A027860 * A203495 A051490 A177748
Adjacent sequences: A059944 A059945 A059946 * A059948 A059949 A059950
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KEYWORD
| easy,nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Feb 14 2001
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