In this chapter, the stability problem is treated in general. In particular, stability of the multiplicative function, logarithmic function, trigonometric functions, vector-valued functions, alternative Cauchy equation, polynomial equation, and quadratic equation is treated. S.M. Ulam, in his famous lecture in 1940 to the Mathematics Club of the University of Wisconsin, presented a number of unsolved problems. This is the starting point of the theory of the stability of functional equations. One of the questions led to a new line of investigation, nowadays known as the stability problems.
KeywordsBanach Space Functional Equation Commutative Semigroup Riesz Space Normed Algebra
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