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Search: id:A060486
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| A060486 |
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Tricoverings of an n-set. |
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+0
3
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| 1, 0, 0, 5, 205, 11301, 904580, 101173251, 15207243828, 2975725761202, 738628553556470
(list; graph; listen)
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OFFSET
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0,4
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COMMENT
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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
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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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EXAMPLE
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There are 1 4-block tricovering, 3 5-block tricoverings and 1 6-block tricovering of a 3-set,(cf. A060487), so a(3)=5.
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CROSSREFS
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Cf. A006095, A060483-A060485, (row sums of) A060487, A060090-A060095, A060069, A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A157389 A128678 A012811 this_sequence A002438 A005333 A162087
Adjacent sequences: A060483 A060484 A060485 this_sequence A060487 A060488 A060489
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KEYWORD
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more,nonn
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AUTHOR
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Vladeta Jovovic (vladeta(AT)eunet.rs), Mar 20 2001
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