Which of the following is the property of mutual information?

S1: Mutual information of the channel is symmetric; that is, I (X; Y) = I (Y; X)

S2: Mutual information of the channel is always non-negative; that is, I (X; Y) ≥ 0

S3: Mutual information of a channel is related to the joint entropy of the channel input and channel output by

I(X; Y) = H(X) + H(Y) − H(X, Y)

S4: H(X, Y) = H(X) + H(Y/X)