Though the letter f is connected with the word "function", but any other letter also can be applied to designation of functional dependence; sometimes even repeat the same letter y: y=y(x). In certain cases write argument and in the form of a badge at function, for example.
Will think wrong however that it is the only way by which function can be set. In the mathematics cases when function is defined without the aid of a formula are frequent. Such is, for example, function E (x) – "the whole part of number x". For example,