On the Ramanujan AGM Fraction, II: The Complex-Parameter Case
Borwein, J. ; Crandall, R.
Experiment. Math., Tome 13 (2004) no. 1, p. 287-295 / Harvested from Project Euclid
The Ramanujan continued fraction {\small $${\cal R}_\eta(a,b) =\,\frac{a}{\displaystyle \eta+\frac{b^2}{\displaystyle \eta +\frac{4a^2}{\displaystyle \eta+\frac{9b^2}{\displaystyle \eta+{}_{\ddots}}}}}$$} ¶ is interesting in many ways; e.g., for certain complex parameters $(\eta, a,b)$ one has an attractive AGM relation ${\cal R}_{\eta} (a,b) + {\cal R}_{\eta}(b,a) = 2 {\cal R}_{\eta} \left((a+b)/2, \sqrt{ab} \right)$. Alas, for some parameters the continued fraction ${\cal R}_{\eta}$ does not converge; moreover, there are converging instances where the AGM relation itself does not hold. To unravel these dilemmas we herein establish convergence theorems, the central result being that ${\cal R}_1$ converges whenever $|a| \not= |b|$. Such analysis leads naturally to the conjecture that divergence occurs whenever $a=b e^{i\phi}$ with $\cos^2\phi \not = 1$ (which conjecture has been proven in a separate work) [Borwein et al. ] We further conjecture that for $a/b$ lying in a certain---and rather picturesque---complex domain, we have both convergence and the truth of the AGM relation.
Publié le : 2004-05-14
Classification:  Continued fractions,  theta functions,  elliptic integrals,  hypergeometric functions,  special functions,  complex variables,  44-A20,  33C05,  11J70
@article{1103749837,
     author = {Borwein, J. and Crandall, R.},
     title = {On the Ramanujan AGM Fraction, II: The Complex-Parameter Case},
     journal = {Experiment. Math.},
     volume = {13},
     number = {1},
     year = {2004},
     pages = { 287-295},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1103749837}
}
Borwein, J.; Crandall, R. On the Ramanujan AGM Fraction, II: The Complex-Parameter Case. Experiment. Math., Tome 13 (2004) no. 1, pp.  287-295. http://gdmltest.u-ga.fr/item/1103749837/