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Search: id:A026945
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| 1, 2, 9, 51, 323, 2188, 15511, 113634, 853467, 6536382, 50852019, 400763223, 3192727797, 25669818476, 208023278209, 1697385471211, 13933569346707, 114988706524270, 953467954114363, 7939655757745265, 66368199913921497
(list; graph; listen)
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OFFSET
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0,2
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COMMENT
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a(n) = sum of the squares of numbers in row n of array T given by A026300.
Number of closed walks of length 2n on the one-way infinite ladder graph starting from (and ending at) a node of degree 2. - Mitch Harris, Mar 06 2004
a(n) = number of ways to connect 2n points labeled 1,2,...,2n in a line with 0 or more noncrossing arcs. For example, with arcs separated by dashes, a(2)=9 counts {} (no arcs), 12, 13, 14, 23, 24, 34, 12-34, 14-23. - David Callan (callan(AT)stat.wisc.edu), Sep 18 2007
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FORMULA
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a(n) = A005043(2n) + A005043(2n+1). - Ralf Stephan, Feb 06 2004
a(n)=sum{k=0..n, C(2n,2k)*C(k)}, C(n)=A000108(n); - Paul Barry (pbarry(AT)wit.ie), Jul 11 2008
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MAPLE
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G:=(1-x-(1-2*x-3*x^2)^(1/2))/(2*x^2): GG:=series(G, x=0, 60): 1, seq(coeff(GG, x^(2*n)), n=1..23);
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CROSSREFS
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Cf. A001006, A099250.
Adjacent sequences: A026942 A026943 A026944 this_sequence A026946 A026947 A026948
Sequence in context: A047069 A020087 A079836 this_sequence A009310 A091319 A003584
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KEYWORD
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nonn,easy
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AUTHOR
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Clark Kimberling (ck6(AT)evansville.edu)
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EXTENSIONS
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Entry revised by N. J. A. Sloane (njas(AT)research.att.com), Nov 16 2004
More terms from Emeric Deutsch (deutsch(AT)duke.poly.edu), Nov 17 2004
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