If is said that X and Y are positively correlated if />LX. Y) > 0, that X and Y are
negatively correlated if p(X, Y) < 0, and that X and Y are uncorrelated if p( X, Y) = 0
It can be seen from Eq. (4.6.2) that Cov(X, Y) and p(X, Y) must have the same sign;
that is, both are positive. or both are negative, or both are zero.