It is evident that at each point inside the circle x^2+y^2≤1,f(x,y) can be factored as in Eq. (3.5.7). However, this same factorization cannot also be satisfied at every point outside this circle. The important feature of this example is that values of X and Y are constrained to lie inside a circle. The joint p.d.f. of X and Y is positive inside the circle and zero outside the circle. Under these conditions, X and Y cannot be independent, because for every given value y of Y, the possible values of X will depend on y. For example, if Y = 0, then X can have any value such that X^2≤ 1; if Y = 1/2, then X must have a value such that X^2≤ 3/4.