Gaussian Multivariate
Assume we have a multivariate normal random variable X = [X1, X2, X3, X4]T whose covariance matrix Σ and inverse covariance matrix Q are:
Σ = [0.71, 0.43, 0.43, -0.43; 0.43, 0.46, 0.26, 0.2; 0.43, 0.26, 0.46, -0.3; -0.43, 0.2, -0.3, 0.5]
Note that Q is simply the inverse of Σ, i.e. Q = Σ^(-1).
Are X3 and X4 correlated? Are X2 and X4 conditionally correlated given the other variables? That is, does cov(X3, X4 | X1, X2) equal to zero?
(c) Please find the Markov blanket of X2. Recall that the Markov blanket of Xi is the set of variables (denoted by Xi;), such that Xi ⊥ {Xj} \ {Xi}, where ⊥ denotes independence. Assume that Y = [Y1, Y2]T is defined by:
Y1 = X1 + X2
Y2 = X3 - X4
Please calculate the covariance matrix of Y.