The geometric optics stability method is extended to a general class of linear advective PDE’s with pseudodifferential bounded perturbation. We give a new short proof of Vishik’s formula for the essential spectral radius. We show that every point in the dynamical spectrum of the corresponding bicharacteristicamplitude system contributes a point into the essential spectrum of the PDE. Generic sp...

The majority of hypertensive patients fall into the borderline and mild groups. A smaller percentage of the hypertensive population falls into the groups with persistent elevations in diastolic blood pressure of 105 mm Hg or higher. However, they are a most important group because treatment has been effective in reducing their high risk of developing major complications. In mild hypertension, t...

In this paper‎, ‎we discuss the essential spectrum of sum of two bounded operators‎ ‎using measure of non strict singularity‎. ‎Based on this new investigation‎, ‎a problem of one-speed neutron transport operator is presented‎.

We characterize the spectrum and essential spectrum of “essentially linear fractional” composition operators acting on the Hardy space H2(U) of the open unit disc U. When the symbols of these composition operators have Denjoy-Wolff point on the unit circle, the spectrum and essential spectrum coincide. Our work permits us to describe the spectrum and essential spectrum of certain associated wei...