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 A052521 Number of pairs of sequences of cardinality at least 3. 0
 0, 0, 0, 0, 0, 0, 720, 10080, 120960, 1451520, 18144000, 239500800, 3353011200, 49816166400, 784604620800, 13076743680000, 230150688768000, 4268249137152000, 83230858174464000, 1703031405723648000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,7 LINKS Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 88 FORMULA E.g.f.: x^6/(-1+x)^2 Recurrence: {a(1)=0, a(0)=0, a(2)=0, a(4)=0, a(3)=0, a(5)=0, (-n^2+3*n+4)*a(n)+(n-5)*a(n+1), a(6)=720} (n-5)*n! MAPLE Pairs spec := [S, {B=Sequence(Z, 3 <= card), S=Prod(B, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); CROSSREFS Sequence in context: A187192 A052792 A052790 * A213876 A052785 A052783 Adjacent sequences:  A052518 A052519 A052520 * A052522 A052523 A052524 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 STATUS approved

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