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Published online by Cambridge University Press: 20 November 2018
For two functions $f$ and
$g$, define
$g\ll f$ to mean that
$g$ satisfies every algebraic differential equation over the constants satisfied by
$f$. The order
$\ll $ was introduced in one of a set of problems on algebraic differential equations given by the late Lee Rubel. Here we characterise the set of
$g$ such that
$g\ll f$, when
$f$ is a given Liouvillian function.