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Combinatorial and modular solutions of some sequences with links to a certain conformal map
Volume 59, no. 2
(2018),
pp. 389–414
DOI: https://doi.org/10.33044/revuma.v59n2a09
Abstract
If $f_n$ is a free parameter, we give a combinatorial closed form
solution of the recursion \[ (n+1)^2 u_ {n+1} -f_n u_n-n^2 u_ {n-1}
=0, \quad n \geq 1, \] and a related generating function. This is
used to give a solution to the Apéry type sequence \[ r_n n^3+r_
{n-1} \left \{ \alpha n^3- \frac{3\alpha}{2} n^2+ \left \{
\frac{\alpha}{2} +2 \theta \right \} n- \theta \right \} +r_ {n-2}
(n-1)^3=0, \quad n \geq 2, \] for certain parameters $ \alpha,
\theta$. We show from another viewpoint two independent solutions of
the last recursion related to certain modular forms associated with a
problem of conformal mapping: Let $f( \tau)$ be a conformal map of a
zero-angle hyperbolic quadrangle to an open half plane with values
$0$, $ \rho$, $1$, $ \infty$ ($0< \rho<1$) at the cusps and
define $t=t( \tau): = \frac{1}{\rho} f( \tau)
\frac{f(\tau)-\rho}{f(\tau)-1} $. Then the function \[ E( \tau)=
\frac{1}{2\pi i} \frac{f'(\tau)}{f(\tau)}
\frac{1}{1-\frac{f(\tau)}{\rho}} \] is a solution, as a generating
function in the variable $t$, of the above recurrence. In other
words, $E( \tau)=r_0+r_1t+r_2 t^2+ \dots$, where $r_0=1$, $r_1=-
\theta$, $ \alpha=2- \frac{4}{\rho} $.
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Published by the Unión Matemática Argentina |