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Search: id:A120918
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| 1, 12, 124, 1212, 11512, 107544, 994236, 9128024, 83400856, 759387964, 6896903064, 62519804504, 565914425336, 5116780986152, 46223426993576, 417279346904792, 3764890593799336, 33953608251139560, 306100904240342268
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OFFSET
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0,2
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FORMULA
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G.f.: A(x) = H(x)*(1-x)/(1-9*x), where H(x) is the g.f. of A120915: H(x) = C(2x)^2*H(x*C(2x)^4), and C(x) is the g.f. of A000108 (Catalan).
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PROGRAM
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(PARI) {a(n)=local(A, F=1+4*x+4*x^2, d=2, G=x, H=1+x, S=ceil(log(n+1)/log(d+1))); for(i=0, n, G=x*subst(F, x, G+x*O(x^n))); for(i=0, S, H=subst(H, x, x*G^d+x*O(x^n))*G/x); A=(x*H-y*subst(H, x, x*y^d +x*O(x^n)))/(x*subst(F, x, y)-y); sum(k=0, 2*n, polcoeff(polcoeff(A, n, x), k, y))}
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CROSSREFS
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Cf. A120914, A120915, A120916, A120917.
Adjacent sequences: A120915 A120916 A120917 this_sequence A120919 A120920 A120921
Sequence in context: A071269 A061088 A125424 this_sequence A039680 A016134 A045507
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KEYWORD
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nonn
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AUTHOR
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Paul D. Hanna (pauldhanna(AT)juno.com), Jul 17 2006
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