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Search: id:A052991
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| A052991 |
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A simple regular expression. |
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+0
1
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| 1, 2, 6, 20, 66, 218, 720, 2378, 7854, 25940, 85674, 282962, 934560, 3086642, 10194486, 33670100, 111204786, 367284458, 1213058160, 4006458938, 13232434974, 43703763860, 144343726554, 476734943522, 1574548557120
(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 1066
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FORMULA
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G.f.: (-1+x+x^2)/(-1+3*x+x^2)
Recurrence: {a(0)=1, a(1)=2, a(n)+3*a(n+1)-a(n+2), a(2)=6}
Sum(-2/13*(3*_alpha-2)*_alpha^(-1-n), _alpha=RootOf(-1+3*_Z+_Z^2))
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MAPLE
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spec := [S, {S=Sequence(Prod(Sequence(Union(Prod(Z, Z), Z)), Union(Z, Z)))}, unlabeled ]: seq(combstruct[count ](spec, size=n), n=0..20);
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CROSSREFS
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Sequence in context: A027061 A083323 A111285 this_sequence A108627 A148475 A148476
Adjacent sequences: A052988 A052989 A052990 this_sequence A052992 A052993 A052994
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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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EXTENSIONS
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More terms from James A. Sellers (sellersj(AT)math.psu.edu), Jun 06 2000
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