NO.PZ2015121802000023答案中说:a zero-variance portfolio can only be constructed if the correlation coeffecient between assets is -1.
NO.PZ2015121802000028答案中倒数第二行提到:if covariance is zero, then correlation is also zero.
对于这两题的答案不是很理解,variance等于0时,相关系数是0还是-1呢?