OFFSET
0,2
COMMENTS
Conjecture: limit |a(n)/n!|^(-1/n) = r exists and is finite with r<0.8...
What is the radius of convergence of the e.g.f. as a power series in x?
r = Pi/4. - Vaclav Kotesovec, Sep 27 2019
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 0..430
FORMULA
E.g.f.: exp(2x)*AGM(1, cosh(2x)).
E.g.f.: exp(2x)*AGM( cosh(x)^2, sqrt(cosh(2x)) ).
EXAMPLE
E.g.f.: A(x) = 1 + 2*x + 6*x^2/2! + 20*x^3/3! + 66*x^4/4! +...
Special value:
A(log(2)/8) = Pi^(3/2)*sqrt(8)/gamma(1/4)^2 = 1.19814023473...
MATHEMATICA
nmax = 20; CoefficientList[Series[E^(4*x)*Pi / (2*EllipticK[1 - E^(-8*x)]), {x, 0, nmax}], x] * Range[0, nmax]! (* Vaclav Kotesovec, Sep 27 2019 *)
PROG
(PARI) {a(n)=n!*polcoeff(agm(1, exp(4*x+x*O(x^n))), n)}
CROSSREFS
KEYWORD
sign
AUTHOR
Paul D. Hanna, Jan 24 2011
STATUS
approved