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A131313
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Number of labeled 4-arch graphs on n nodes.
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0
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0, 0, 0, 1, 1, 15, 1085, 216230, 92550276, 74358276300, 102660061574400, 228311514581611725, 777903709095959606875, 3900045557544532389044051, 27829438251904221299882526375, 274842860343937013061411903607000, 3668195394384971166861021308091920000
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OFFSET
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1,6
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LINKS
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FORMULA
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The number of labeled 4-arch graphs with n>5 nodes is given by f(n,4,n-4-1,0,4) 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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