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Search: id:A052516
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| A052516 |
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Number of pairs of sets of cardinality at least 3. |
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+0
1
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| 0, 0, 0, 0, 0, 0, 20, 70, 182, 420, 912, 1914, 3938, 8008, 16172, 32526, 65262, 130764, 261800, 523906, 1048154
(list; graph; listen)
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OFFSET
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0,7
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LINKS
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INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 82
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FORMULA
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E.g.f.: exp(x)^2-2*exp(x)-2*x*exp(x)-exp(x)*x^2+1+2*x+2*x^2+x^3+1/4*x^4
Recurrence: {a(1)=0, a(2)=0, (2*n+2)*a(n)+(-3*n+1)*a(n+1)+(n-1)*a(n+2), a(3)=1/16*_C[0], a(4)=5/16*_C[0], a(5)= _C[0], a(6)=20+21/8*_C[0]}
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MAPLE
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Pairs spec := [S, {S=Prod(B, B), B=Set(Z, 3 <= card)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
with (combstruct):ZL:=[S, {S=Sequence(U, card=r), U=Set(Z, card>=3)}, labeled]: seq(count(subs(r=2, ZL), size=m), m=0..20); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 09 2007
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CROSSREFS
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Sequence in context: A153728 A071395 A053741 this_sequence A083873 A143854 A139241
Adjacent sequences: A052513 A052514 A052515 this_sequence A052517 A052518 A052519
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KEYWORD
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easy,nonn
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AUTHOR
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encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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