  An intense X wave propagating perpendicularly to dc magnetic field is
unstable with respect to a parametric decay into an electron Bernstein wave and
a lower-hybrid wave. A modified theory of this effect is proposed that extends
to the high-intensity regime, where the instability rate $\gamma$ ceases to be
a linear function of the incident-wave amplitude. An explicit formula for
$\gamma$ is derived and expressed in terms of cold-plasma parameters. Theory
predictions are in reasonable agreement with the results of the
particle-in-cell simulations presented in a separate publication.
