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A121649
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G.f.: A(x) = 1/(1 - x*B(x^2)), where B(x) = Sum_{n>=0} a(n)^2*x^n is the g.f. of A121648.
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3
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1, 1, 1, 2, 3, 5, 8, 16, 27, 51, 89, 170, 300, 564, 1008, 1972, 3563, 6847, 12483, 24340, 44583, 86071, 158600, 309554, 572548, 1108068, 2057792, 4003278, 7451924, 14415482, 26913176, 52545636, 98321435, 190858943, 358017691, 698449146
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OFFSET
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0,4
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LINKS
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FORMULA
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G.f. satisfies: A(x)*A(-x) = ( A(x) + A(-x) )/2. - Paul D. Hanna, Aug 14 2006
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EXAMPLE
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A(x) = 1 + x + x^2 + 2*x^3 + 3*x^4 + 5*x^5 + 8*x^6 + 16*x^7 + 27*x^8 +...
The coefficients of 1 - 1/A(x) equal the square of each term:
1/A(x) = 1 - x - x^3 - x^5 - 4*x^7 - 9*x^9 - 25*x^11 - 64*x^13 - 256*x^15 -... - a(n)^2*x^(2*n+1) -...
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PROG
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(PARI) {a(n)=local(B); if(n==0, 1, B=sum(k=0, n\2, a(k)^2*x^(2*k)); polcoeff(1/(1-x*B+x*O(x^n)), n))}
(PARI) {a(n)=local(A, m); if(n<0, 0, m=1; A=1+x+O(x^2); while(m<=n, m*=2; A=1/(1-x*sum(k=0, m-1, polcoeff(A, k)^2*x^(2*k), O(x^(2*m))))); polcoeff(A, n))} /* Michael Somos, Aug 18 2006 */
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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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