Generalization of Ulam stability problem for Euler–Lagrange quadratic mappings

In 1968 S.M. Ulam proposed the problem: “When is it true that by changing a little the hypotheses of a theorem one can still assert that the thesis of the theorem remains true or approximately true?” In 1978 P.M. Gruber proposed the Ulam type problem: “Suppose a mathematical object satisfies a certain property approximately. Is it then possible to approximate this object by objects, satisfying the property exactly?” In this paper we solve the generalized Ulam stability problem for non-linear Euler–Lagrange quadratic mappings satisfying approximately a mean equation and an Euler–Lagrange type functional equations in quasi-Banach spaces and p-Banach spaces.