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A060051
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Number of n-block r-bicoverings.
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21
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1, 0, 0, 2, 79, 82117, 4936900199, 27555467226181396, 20554872166566046969648895, 2786548447182420815380482508924733911, 89607283195144164483079065133414172790220498449945
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,4
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COMMENTS
| A bicovering is an r-bicovering if the intersection of every two blocks contains at most one element.
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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
| E.g.f. for number of n-block r-bicoverings of a k-set is exp(-x-1/2*x^2*y)*Sum_{i=0..inf} (1+y)^binomial(i, 2)*x^i/i!.
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EXAMPLE
| There are 2 3-block r-bicoverings: {{1},{2},{1,2}} and {{1,2},{1,3},{2,3}}.
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CROSSREFS
| Cf. A060052, A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A166052 A045484 A113152 * A100421 A195000 A073499
Adjacent sequences: A060048 A060049 A060050 * A060052 A060053 A060054
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KEYWORD
| easy,nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Feb 15 2001
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