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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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0, 0, 1, 1, 10, 380, 34405, 5919536, 1709074584, 764754595200, 501266271119625, 461131039993333600, 575567346874153958896, 948234014731772610333856, 2015213091846867369590804125, 5418520468131267088808607104000, 18128426763624683428523183481856000
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OFFSET
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1,5
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LINKS
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FORMULA
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The number of labeled 3-arch graphs 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 below.
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PROG
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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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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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EXTENSIONS
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STATUS
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approved
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