(
Mathematics:)
A
functional is a
function from a
space of functions to
scalars. Usually
linear functionals are intended, and the prefix "
linear" may even be dropped.
An example of a linear functional on the space C0(R) of continuous functions on R that tend to 0 is φ(f)=f(0) ("substitution"). An example of a nonlinear functional is the variation of f.