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Search: id:A060069
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| A060069 |
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Number of n-block T_0-tricoverings. |
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+0
19
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| 1, 0, 0, 0, 2, 82194, 9185157387760082, 5573096894405951375691132323893805593, 47933892393105239218152796441416602126447041437452022947424986090407628
(list; graph; listen)
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OFFSET
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0,5
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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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FORMULA
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E.g.f. for n-block T_0-tricoverings of a k-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. A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A060895 A100266 A071067 this_sequence A114950 A003840 A122540
Adjacent sequences: A060066 A060067 A060068 this_sequence A060070 A060071 A060072
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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 19 2001
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