  The problem of finding a nonzero solution of a linear recurrence $Ly = 0$
with polynomial coefficients where $y$ has the form of a definite
hypergeometric sum, related to the Inverse Creative Telescoping Problem of
[14][Sec. 8], has now been open for three decades. Here we present an algorithm
(implemented in a SageMath package) which, given such a recurrence and a
quasi-triangular, shift-compatible factorial basis $\mathcal{B} = \langle
P_k(n)\rangle_{k=0}^\infty$ of the polynomial space $\mathbb{K}[n]$ over a
field $\mathbb{K}$ of characteristic zero, computes a recurrence satisfied by
the coefficient sequence $c = \langle c_k\rangle_{k=0}^\infty$ of the solution
$y_n = \sum_{k=0}^\infty c_kP_k(n)$ (where, thanks to the quasi-triangularity
of $\mathcal{B}$, the sum on the right terminates for each $n \in \mathbb{N}$).
More generally, if $\mathcal{B}$ is $m$-sieved for some $m \in \mathbb{N}$, our
algorithm computes a system of $m$ recurrences satisfied by the $m$-sections of
the coefficient sequence $c$. If an explicit nonzero solution of this system
can be found, we obtain an explicit nonzero solution of $Ly = 0$.
