In many areas of mathematics one can ask the following question:
When is it true that a mathematical object satisfying a certain property
approximately must be close to an object satisfying the property exactly.
In functional equations, one can ask when the solutions of a functional
equation diering slightly from a given one must be close to the solution
of a given equation. The stability theory for functional equation was
initiated by a problem of S. M. Ulam in 1940 (see Ulam (1960)).