2019-01-27[Stay Sharp] Covariance

Covariance is a measure of the joint variability of two random variable variables.

represents the covariance between two jointly distributed real-valued random variables and

If the random variable pair can take on the values for with equal probabilities , then the covariance can be written as:

if probabilities are not equal, the covariance could be:

Ref

https://en.wikipedia.org/wiki/Covariance