  Relation between two properties of linear difference equations with infinite
delay is investigated: (i) exponential stability, (ii) $\l^p$-input
$\l^q$-state stability (sometimes is called Perron's property). The latter
means that solutions of the non-homogeneous equation with zero initial data
belong to $\l^q$ when non-homogeneous terms are in $\l^p$. It is assumed that
at each moment the prehistory (the sequence of preceding states) belongs to
some weighted $\l^r$-space with an exponentially fading weight (the phase
space). Our main result states that (i) $\Leftrightarrow$ (ii) whenever $(p,q)
\neq (1,\infty)$ and a certain boundedness condition on coefficients is
fulfilled. This condition is sharp and ensures that, to some extent,
exponential and $\l^p$-input $\l^q$-state stabilities does not depend on the
choice of a phase space and parameters $p$ and $q$, respectively. $\l^1$-input
$\l^\infty$-state stability corresponds to uniform stability. We provide some
evidence that similar criteria should not be expected for non-fading memory
spaces.
