Whereas standard Riemann integration sums a function f (x) over a continuous range of values of x, functional integration sums a functional G [ f ], which can be thought of as a "function of a function" over a continuous range (or space) of functions f. Most functional integrals cannot be evaluated exactly but must be evaluated using perturbation methods. The formal definition of a functional integral is ∫ ∫ G [ f ] D [ f ] ≡ ≡ ∫ ∫ R ⋯ ⋯ ∫ ∫ R G [ f ] ∏ ∏ x d f ( x ) .