Evaluation of the stability of some inverse problems solutions for integro-differential equations

The paper investigates the stability of inverse problems solutions for two integro-differential hyperbolic equations. Theorems of existence and uniqueness of these solutions (in the small) have been obtained and published earlier by author. Thus only stability problems of these solutions are considered in this paper. In Theorem 1 we prove conditional stability of the solution of the following inverse problem: determine the kernel of the integral for integro-differential equation
$$ u_{tt}=u_{xx}-\int_0^tk(\tau)u(x,t-\tau)\,d\tau,\qquad (x,t)\in\mathbb R\times\mathbb R_+, $$
with initial data $u\big|_{t=0}=0$, $u_t\big|_{t=0}=\delta(x)$, and additional information about the direct problem solution $u(0,t)=f_1(t)$, $u_x(0,t)=f_2(t)$. The inverse problem is replaced by an equivalent system of integral equations for the unknown functions. To prove the theorem the method of successive approximations is used. Next, the method of estimating the integral equations and Gronwall's inequality are used.
In a similar manner we prove Theorem 2. It is devoted to estimating the conditional stability of the solution of kernel determination problem for the same integro-differential equation in a bounded domain with respect to $x$, $x\in(0,l)$, with initial data $u\big|_{t=0}=0$, $u_t\big|_{t=0}=\delta'(x)$, and boundary conditions $(u_x-hu)\big|_{x=0}=0$, $(u_x+Hu)\big|_{x=l}=0$, $t>0$. In this case the additional information about the direct problem solution is given as $u(0,t)=f(t)$, $t\geqslant0$. Here $h$ and $H$ are finite real numbers.