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A005784
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Number of 4-covers of an n-set.
(Formerly M3535)
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3
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4, 17, 65, 230, 736, 2197, 6093, 15864, 38960, 90837, 202005, 430577, 883057, 1748909, 3355213, 6252575, 11345602, 20089514, 34778306, 58964020, 98053576, 160151566, 257229974, 406739271, 633795181, 974126408, 1477999320
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history;
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OFFSET
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1,1
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REFERENCES
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R. J. Clarke, Covering a set by subsets, Discrete Math., 81 (1990), 147-152.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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Table of n, a(n) for n=1..27.
Vladeta Jovovic, Binary matrices up to row and column permutations
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FORMULA
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G.f.: (x^20 - x^19 + 4*x^18 + 9*x^17 + 23*x^16 + 39*x^15 + 90*x^14 + 131*x^13 + 204*x^12 + 238*x^11 + 252*x^10 + 238*x^9 + 204*x^8 + 131*x^7 + 90*x^6 + 39*x^5 + 23*x^4 + 9*x^3 + 4*x^2 - x + 1)/((1 - x^4)^3*(1 - x^3)^4*(1 - x^2)^3*(1 - x)^5).
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CROSSREFS
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Cf. A006148, A005783, A005785.
Sequence in context: A122231 A119916 A209375 * A095252 A181410 A102207
Adjacent sequences: A005781 A005782 A005783 * A005785 A005786 A005787
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KEYWORD
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easy,nonn
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AUTHOR
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N. J. A. Sloane.
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EXTENSIONS
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More terms from Vladeta Jovovic, Jun 03 2000
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STATUS
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approved
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