Show that the four products X*1(r)X2(r), X1(r)X*2(r), X1(r)X2(r) and X*1(r)X*2(r) are integrable (summable) functions given that X1(r) and X2(r)are two square intergral functions. (X1(r) is Psi subscript 1 function of r and X*1(r) is the conjugate of X1(r))
