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A087218
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Satisfies A(x) = 1 + x*A(x)*f(x)^2, where f(x) = Sum_{k>=0} x^((3^n-1)/2) and f(x)^2 = 2 - f(x^2) + 2*Sum_{n>0} x^A023745(n).
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2
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1, 1, 3, 6, 13, 30, 66, 147, 327, 726, 1614, 3588, 7974, 17725, 39399, 87573, 194655, 432669, 961716, 2137659, 4751490, 10561392, 23475378, 52179987, 115983270, 257802273, 573031011, 1273706934, 2831137095, 6292921101, 13987615113
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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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a(n) = A078932(2n). a(m) = 1 (mod 3) when m = (3^n - 1)/2, otherwise a(m) = 0 (mod 3).
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EXAMPLE
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Given f(x) = 1 + x + x^4 + x^13 + x^40 + x^121 + ... so that f(x)^2 = 1 + 2x + x^2 + 2x^4 + 2x^5 + x^8 + 2*x^13 + ... then A(x) = 1 + x*A(x)*(1 + 2x + x^2 + 2x^4 + 2x^5 + x^8 + ...) = 1 + x + 3x^2 + 6x^3 + 13x^4 + 30x^5 + ...
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PROG
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(PARI) a(n)=local(A, m); if(n<1, 1, m=1; A=1+O(x); while(m<=2*n, m*=3; A=1/(1/subst(A, x, x^3)-x)); polcoeff(A, 2*n));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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