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A055195 Number of 3-covers of an unlabeled n-set. 0
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; text; internal format)
OFFSET

2,2

COMMENTS

Covers are defined in proper sense, i.e. without empty sets or multiple sets.

LINKS

Table of n, a(n) for n=2..36.

Eric Weisstein's World of Mathematics, Covers

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).

CROSSREFS

Cf. A005783.

Sequence in context: A272997 A273113 A273177 * A273243 A273297 A273268

Adjacent sequences:  A055192 A055193 A055194 * A055196 A055197 A055198

KEYWORD

easy,nonn

AUTHOR

Vladeta Jovovic, Jun 30 2000

EXTENSIONS

More terms from James A. Sellers, Jul 04 2000

STATUS

approved

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Last modified April 6 15:23 EDT 2020. Contains 333276 sequences. (Running on oeis4.)