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Search: id:A131311
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| A131311 |
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Number of labeled 2-arch graphs on n nodes. |
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+0
5
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| 0, 1, 1, 6, 100, 3285, 177471, 14188888, 1569185136, 229087571625
(list; graph; listen)
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OFFSET
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1,4
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LINKS
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Saverio Caminiti and Emanuele G. Fusco, On the Number of Labeled k-arch Graphs, Journal of Integer Sequences, Vol 10 (2007), Article 07.7.5
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FORMULA
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The number of labeled 2-arch graps with n>3 nodes is given by f(n,2,n-2-1,0,2) where f is the recursive function described by the pari-gp code attached below.
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PROGRAM
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(PARI) f(n, k, i, u, j)={ local(s=0); if (i==1, binomial(n-u, j)*binomial(u, k-j), for (c=0, min(k, n-(i-1)-(u+j)), s+=f(n, k, i-1, u+j, c) ); binomial(n-u, j)*binomial(u, k-j)*s ) }
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CROSSREFS
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Cf. A098721-A098724, A131312-A131315.
Sequence in context: A064753 A138913 A127636 this_sequence A098721 A078629 A012497
Adjacent sequences: A131308 A131309 A131310 this_sequence A131312 A131313 A131314
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KEYWORD
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nonn
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AUTHOR
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Saverio Caminiti and Emanuele G. Fusco (fusco(AT)di.uniroma1.it), Sep 18 2007
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