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A060483
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Number of 5-block tricoverings of an n-set.
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9
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3, 57, 717, 7845, 81333, 825237, 8300757, 83202645, 832809813, 8331237717, 83324947797, 833299785045, 8333199127893, 83332796486997, 833331185898837, 8333324743497045, 83333298973791573, 833333195894773077
(list; graph; refs; listen; history; internal format)
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OFFSET
| 3,1
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COMMENTS
| A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering.
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FORMULA
| a(n)=(1/5!)*(10^n-15*4^n+45*2^n-40); generally, e.g.f. for k-block tricoverings of an n-set is exp(-x+x^2/2+(exp(y)-1)*x^3/3)*Sum_{k=0..inf}x^k/k!*exp(-1/2*x^2*exp(k*y))*exp(binomial(k, 3)*y).
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CROSSREFS
| Cf. A006095, A060484-A060487, A060090-A060095, A060069, A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A032696 A131466 A171356 * A139746 A157929 A053725
Adjacent sequences: A060480 A060481 A060482 * A060484 A060485 A060486
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KEYWORD
| nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Mar 20 2001
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