Does Covariance Remain Unchanged Under Variable Transformations?

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The discussion centers on the behavior of covariance under variable transformations, specifically when two random variables X1 and Y1 have Cov(X1,Y1) = 0. It is established that this does not imply Cov(X2,Y2) = 0 for transformed variables X2 = g(X1) and Y2 = g(Y1) using a continuous function g. An example illustrates that even if X1 and Y1 are uncorrelated, their transformations X2 and Y2 can exhibit non-zero covariance, particularly when the transformation is quadratic, as shown with the joint distribution of X and Y.

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PAHV
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Let X1 and Y1 be two random variables. We have Cov(X1,Y1) = 0. Does this extend to any transformation X2 = g(X1) and Y2 = g(Y1), such that Cov(X2,Y2)? Here, g is a continuous function. For example, if we set X2 = X1^2 and Y2 = Y1^2. Do we then from Cov(X1,Y1) = 0 that Cov(X1^2,Y1^2) = 0?
 
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No. For example, if X1 and Y1 are uncorrelated but not independent, then your X2 and Y2 may have a non-zero correlation.
 
For example, try random variables X and Y with the joint distribution given by:

P(X=-2,Y=3) = 1/4
P(X=-1,Y=2) = 1/4
P(X=1,Y=2) = 1/4
P(X=2,Y=3) = 1/4

and transform by g(w) = w^2
 

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