f(x) is a function that satisfies the two properties:

* Additivity: 1=''f''(''x'' + ''y'') = ''f''(''x'') + ''f''(''y'').
* Homogeneity of degree 1: 1=''f''(?''x'') = ? ''f''(''x'') for all ?.

These properties are known as the superposition principle. In this definition, x is not necessarily a real number, but can in general be an element