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A055195
Number of 3-covers of an unlabeled n-set.
2
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
OFFSET
2,2
COMMENTS
Covers are defined in proper sense, i.e. without empty sets or multiple sets.
LINKS
Eric Weisstein's World of Mathematics, Covers
Index entries for linear recurrences with constant coefficients, signature (3,-1,-3,-1,3,6,-6,-3,1,3,1,-3,1).
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).
MATHEMATICA
Drop[CoefficientList[Series[x^2(x^11-x^10-3x^9+4x^7+6x^6-3x^5-5x^4-3x^3+ 3x^2+6x+1)/ ((1-x^3)^2(1-x^2)^2(1-x)^3), {x, 0, 40}], x], 2] (* or *) LinearRecurrence[ {3, -1, -3, -1, 3, 6, -6, -3, 1, 3, 1, -3, 1}, {1, 9, 29, 72, 154, 294, 522, 877, 1406, 2172, 3255, 4746, 6763}, 40] (* Harvey P. Dale, Jul 18 2021 *)
CROSSREFS
Column 3 of A055130.
Cf. A005783.
Sequence in context: A272997 A273113 A273177 * A343497 A273243 A273297
KEYWORD
nonn,easy
AUTHOR
Vladeta Jovovic, Jun 30 2000
EXTENSIONS
More terms from James A. Sellers, Jul 04 2000
STATUS
approved