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Search: id:A060070
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| A060070 |
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Number of T_0-tricoverings of an n-set. |
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+0
21
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| 1, 0, 0, 5, 175, 9426, 751365, 84012191, 12644839585, 2479642897109, 617049443550205, 190678639438170502, 71860665148118443795, 32527628234581386962713, 17454341903042193018433239
(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. A covering of a set is a T_0-covering if for every two distinct elements of the set there exists a block of the covering containing one but not the other element.
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REFERENCES
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I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
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LINKS
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T_0-tricoverings of a 4-set
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FORMULA
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E.g.f. for k-block T_0-tricoverings of an n-set is exp(-x+1/2*x^2+1/3*x^3*y)*Sum_{i=0..inf}(1+y)^binomial(i, 3)*exp(-1/2*x^2*(1+y)^i)*x^i/i!.
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CROSSREFS
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Cf. A060069, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A116629 A139986 A123111 this_sequence A027873 A052272 A111515
Adjacent sequences: A060067 A060068 A060069 this_sequence A060071 A060072 A060073
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KEYWORD
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nonn
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AUTHOR
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Vladeta Jovovic (vladeta(AT)Eunet.yu), Feb 21 2001
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