Where g_1 is a nonnegative is function of x alone and g_2 is a nonnegative function of y alone. In other words, it is necessary and sufficient that, for all values of x and y, f can be factored into the product of an arbitrary nonnegative function of x and an arbitrary nonnegative function of y. However, it should be emphasized that, just as in Eq. (3.5.5), the factorization in Eq. (3.5.7) must be satisfied for all values of x and y(-∞