where "V" is the function space and formula_6 the L2 inner product, and formula_7 the Wiener measure. The basic assumption on "S" is that it should be self-adjoint, and have discrete spectrum λ1, λ2, λ3, … with a corresponding set of eigenfunctions "f"1, "f"2, "f"3, … which are complete in L2 (as would, for example, be the case for the second derivative operator on a compact interval Ω). This roughly means all functions φ can be written as linear combinations of the functions "f""i":