  If $f$ is a transcendental entire function with only algebraic singularities
we calculate the Ruelle operator of $f$. Moreover, we prove both (i) if $f$ has
a summable critical point, then $f$ is not structurally stable under certain
topological conditions and (ii) if all critical points of $f$ belonging to
Julia set are summable, then there exists no invariant lines fields in the
Julia set.
