Often the one particle space H is given as L 2 ( X , μ μ ) , the space of square-integrable functions on a space X with measure μ μ (strictly speaking, the equivalence classes of square integrable functions where functions are equivalent if they differ on a set of measure zero). The typical example is the free particle with H = L 2 ( R 3 , d 3 x ) the space of square integrable functions on three-dimensional space. The Fock spaces then have a natural interpretation as symmetric or anti-symmetric square integrable functions as follows.