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A120419
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E.g.f. A(x) satisfies A(x) = (1 + (Integral A(x) dx)^2 / 2)^2.
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1
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1, 2, 22, 584, 28384, 2190128, 245762848, 37788392576, 7625538720256, 1954588198280192, 620259836756837632, 238698984906300222464, 109521341941344601083904, 59065100769855968517951488, 36990397033719114096675954688
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OFFSET
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0,2
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COMMENTS
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Previous name was: A mysterious sequence.
This is based on the derivatives of the real function g(x) := -1/f(x)^2:
The algorithm for the sequence is as follows.
(1) Dj = 0, for each j, when j is odd (j=2k+1); (odd derivatives are null)
(3) D2 = -1*f(a)^-2; then b1 = 1; (the 2nd derivative)
(4) D4 = -2*f(a)^-5; (the 4th derivative) So b2 = 2;
(5) D6 = -22*f(a)^-8; (the 6th derivative) So b3 = 22;
(6) D8 = -584*f(a)^-11 (the 8th derivative) So b4 = 584;
(8) D10= -28384*f(a)^-14 (the 10th derivative) So b5 = 28384; and so on...
(n) D2n= -bn*f(a)^-(3n-1) (the 2n-th derivative) on general bn is unknown.
a(n) = [x^(2n) / (2n)!] A(x). A(-x) = A(x). - Michael Somos, Aug 26 2014
Number of 2-bundled bilabeled increasing trees with 2n labels. - Markus Kuba, Nov 18 2014
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LINKS
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FORMULA
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E.g.f. A(x) satisfies: A(x) = (1 + Integral (A(x) * Integral A(x) dx) dx)^2. - Paul D. Hanna, Aug 26 2014
E.g.f. A(x) satisfies: A'(x) = 2*A(x)^(3/2) * Integral A(x) dx. - Paul D. Hanna, Aug 26 2014
Note that the e.g.f. for Euler numbers (A000364) satisfies G(x) = 1 + Integral (G(x) * Integral G(x)^2 dx) dx when G(x) = 1/cos(x). - Paul D. Hanna, Aug 26 2014
E.g.f.: (1 + Series_Reversion( sqrt(2)*( atan(x) + x/(1+x^2) )/2 )^2 )^2. - Paul D. Hanna, Aug 26 2014, after rewriting a formula due to Robert Israel.
E.g.f. A(x) satisfies A(x) = (1 + (Integral A(x) dx)^2 / 2)^2. - Michael Somos, Aug 26 2014
E.g.f. (for offset 1) T=T(z) satisfies T''=1/(1-T)^2; an implicit equation for T is 2*(arcsin(sqrt(T))+sqrt(T(1-T)))=z^2. - Markus Kuba Nov 18 2014
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EXAMPLE
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E.g.f.: A(x) = 1 + 2*x^2/2! + 22*x^4/4! + 584*x^6/6! + 28384*x^8/8! +...
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MATHEMATICA
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a[ n_] := If[ n < 1, Boole[n == 0], With[{m = 2 n - 1}, m! SeriesCoefficient[ InverseSeries[ Integrate[ Series[ (1 + x^2/2)^-2, {x, 0, m}], x]], {x, 0, m}]]]; (* Michael Somos, Aug 26 2014 *)
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PROG
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(PARI) {a(n)=local(A=1+x); for(i=1, n, A=(1+intformal(A*intformal(A +x*O(x^n))))^2 ); n!*polcoeff(A, n)}
(PARI) {a(n)=local(A); A=(1 + serreverse( sum(m=1, n\2+1, (-1/2)^(m-1) * m * x^(2*m-1) / (2*m-1)) +x^2*O(x^n) )^2/2)^2; n!*polcoeff(A, n)}
(PARI) {a(n) = if( n<1, n==0, n*=2; (n-1)! * polcoeff( serreverse( intformal( (1 + x^2 / 2 + O(x^n))^-2)), n-1))}; /* Michael Somos, Aug 26 2014 */
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Robert Wackensack (wackensack(AT)hotmail.com), Jul 09 2006
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EXTENSIONS
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STATUS
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approved
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