What does knowing Y tell us about X?
This Y↔X relationship is called mutual information.
Which is,
"reduction in uncertainty of X given the knowledge of Y".
There are at least two approaches to deriving the mutual information:

Shannon's approach (using Entropy Algebra)
I(X; Y) = H(X) − H(XY) 
Statistical approach (using Probability distribution).
Statistical approach of deriving I(X; Y) and also showing I(X; Y) = I(Y; X).
Since, mutual information is the relative entropy between the joint distribution and the product distribution p(x_{i}) p(y_{j}),
Thus,
Partial mutual information
From the above (statistical) expression of I(X; Y)
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