However, in most cases the functions f (x) can be written in terms of an infinite series of orthogonal functions such as f ( x ) = f n H n ( x ) , and then the definition becomes ∫ ∫ G [ f ] D [ f ] ≡ ≡ ∫ ∫ R ⋯ ⋯ ∫ ∫ R G ( f 1 ; f 2 ; … … ) ∏ ∏ n d f n ,