Fractional Telegraph equation with the Riemann-Liouville derivative

The Telegraph equation $({\mathrm{\partial }}_t^{\rho }{)}^{2}u(x,t)+2\alpha {\mathrm{\partial }}_t^{\rho }u(x,t)-u_{xx}(x,t)=f(x,t)$, where $0<t\le T$ and $0<\rho <1$, with the Riemann-Liouville derivative is considered. Existence and uniqueness theorem for the solution to the problem under consideration is proved. Inequalities of stability are obtained. The applied method allows us to study a similar problem by taking instead of $d^2\mathrm{/}d{x}^2$ an arbitrary elliptic differential operator $A(x,D)$, having a compact inverse.