ABSTRACT
Perhaps the most common problems in applied mathematics are various kinds of differential equations, since the classical formulations from physics or chemistry are of this nature. The ways of solving these equations are of course many, but one very common method is to reformulate the differential equation into an integral equation. Or, equivalently, instead of formulating the problem as a differential equation, one could from the very start formulate it as an integral equation. The first operators to be considered (apart from matrices) were the integral operators stemming from integral equations of mathematical physics, and it is not wrong to say that the study of integral operators gave birth to modern functional analysis.
