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A131312
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Number of labeled 3-arch graphs on n nodes.
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1
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OFFSET
| 1,5
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LINKS
| 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
| The number of labeled 3-arch graps with n>4 nodes is given by f(n,3,n-3-1,0,3) where f is the recursive function described by the pari-gp code attached below.
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PROG
| (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
| Cf. A098721-A098724, A131311-A131315.
Sequence in context: A117312 A200804 A000591 * A055733 A203774 A024136
Adjacent sequences: A131309 A131310 A131311 * A131313 A131314 A131315
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KEYWORD
| nonn
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AUTHOR
| Saverio Caminiti and Emanuele G. Fusco (fusco(AT)di.uniroma1.it), Sep 18 2007
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EXTENSIONS
| Edited by N. J. A. Sloane (njas(AT)research.att.com), Oct 02 2007
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