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A052751
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A simple grammar.
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4
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1, 1, 4, 19, 107, 647, 4167, 27847, 191747, 1349743, 9671316, 70297105, 517079157, 3841701488, 28787546360, 217317367487, 1651144126659, 12616570941114, 96891439504019, 747452640586114, 5789461514134881
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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0,3
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LINKS
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FORMULA
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G.f.: A(x) = exp(A(x)^3*x + A(x^2)^3*x^2/2 + A(x^3)^3*x^3/3 +...), A(0)=1; also, A(x)^3 = Sum_{n>=0} A006964(n+1)*x^n. - Paul D. Hanna, Jul 13 2006
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MAPLE
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spec := [S, {S=Set(B), B=Prod(S, S, S, Z)}, unlabeled]: seq(combstruct[count](spec, size=n), n=0..20);
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PROG
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(PARI) {a(n)=local(A=1+x+x*O(x^n)); if(n==0, 1, for(i=1, n, A=exp(sum(k=1, n, subst(x*A^3, x, x^k+x*O(x^n))/k))); polcoeff(A, n, x))} \\ Paul D. Hanna, Jul 13 2006
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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STATUS
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approved
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