NO.PZ2017092702000072
问题如下：
Given a portfolio of five stocks, how many unique covariance terms, excluding variances, are required to calculate the portfolio return variance?
选项：
A.10
B.20
C.25
解释：
A is correct.
A covariance matrix for five stocks has 5 × 5 = 25 entries. Subtracting the 5 diagonal variance terms results in 20 off-diagonal entries. Because a covariance matrix is symmetrical, only 10 entries are unique (20/2 = 10)
根据五个股票两两组合，去掉自己和自己组合，一共是5*4/2=10种
是要求这个portfolio的variance，那就应该知道五个数的variance, 还有五个数两两组合的covariance。
covariance是10个，但是不是还有五个variance吗，应该是15个才对啊？
