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A055195
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Number of 3-covers of an unlabeled n-set.
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0
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1, 9, 29, 72, 154, 294, 522, 877, 1406, 2172, 3255, 4746, 6763, 9445, 12953, 17480, 23254, 30528, 39604, 50823, 64568, 81276, 101443, 125610, 154395, 188479, 228611, 275622, 330432, 394032, 467524, 552103, 649064, 759818, 885901
(list; graph; refs; listen; history; internal format)
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OFFSET
| 2,2
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COMMENTS
| Covers are defined in proper sense, i.e. without empty sets or multiple sets.
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LINKS
| Eric Weisstein's World of Mathematics, Covers
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FORMULA
| G.f.: x^2*(x^11-x^10-3*x^9+4*x^7+6*x^6-3*x^5-5*x^4-3*x^3+3*x^2+6*x+1)/((1-x^3)^2*(1-x^2)^2*(1-x)^3).
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CROSSREFS
| Cf. A005783.
Sequence in context: A129397 A136392 A162263 * A024922 A193004 A198645
Adjacent sequences: A055192 A055193 A055194 * A055196 A055197 A055198
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KEYWORD
| easy,nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Jun 30 2000
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EXTENSIONS
| More terms from James A. Sellers (sellersj(AT)math.psu.edu), Jul 04 2000
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