Let X1 and X2 be jointly distributed random variables possessing finite second moments. State conditions under which it is possible to find 2 uncorrelated random variables, Y1 and Y2 which are linear combinations of X1 and X2 (that is, Y1= a11X1 + a12X2 and Y2 = a21X1 + a22X2 for some constants an, a12, a21, a22 and Cov [Y1, Y2] = 0).