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Search: id:A052555
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| A052555 |
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A simple regular expression in a labeled universe. |
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+0
1
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| 1, 2, 10, 72, 696, 8400, 121680, 2056320, 39715200, 862928640, 20832940800, 553246848000, 16027872537600, 503031194265600, 17001946241280000, 615694938034176000, 23782705115000832000, 976080997055324160000
(list; graph; listen)
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OFFSET
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0,2
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LINKS
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INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 496
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FORMULA
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E.g.f.: -1/(-1+2*x+x^2)
Recurrence: {a(0)=1, a(1)=2, (-2-n^2-3*n)*a(n)+(-4-2*n)*a(n+1)+a(n+2)}
Sum(1/4*(1+_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^2))*n!
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MAPLE
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spec := [S, {S=Sequence(Union(Z, Z, Prod(Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
with(combstruct):ZL:=[T, {T=Union(Z, Prod(Epsilon, Z, T), Prod(T, Z, Epsilon), Prod(T, Z, Z))}, labeled]:seq(count(ZL, size=i)/i, i=1..18); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Dec 16 2007
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CROSSREFS
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Sequence in context: A118752 A060842 A111554 this_sequence A084844 A088189 A001395
Adjacent sequences: A052552 A052553 A052554 this_sequence A052556 A052557 A052558
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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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