# Perspective function

Hi,
During the implementation of a perspective function in CVX I got an error. The function is x log(1+ y/x) which is concave.
I think it might be because of the DCP ruleset, but then I am wondering how can I add this function to CVX.
Thanks

How about -rel_entr( x, x + y )? After all,
$$x\log (1+y/x) = x\log((x+y)/x) = - x \log (x/(x+y)) = - f_{\text{rel_entr}}(x,x+y)$$
Please remember that support for logarithms, exponentials, and entropy functions relies on the experimental successive approximation system in CVX.

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Is there anyway to implement the perspective function of log det function (x\log|I+\frac Y x|, x \geq 0,Y \in S^+) in cvx?

@jmiao I believe this can be done using CVXQUAD.https://github.com/hfawzi/cvxquad/9 , which supports quantum_rel_entr(X,Y) defined as trace(X*(logm(X)-logm(Y))) .

-quantum_rel_entr(x*eye(n), x*eye(n) + Y) = x*log(det(eye(n) + Y/x)) , which is the matrix (quantum) generalization of @mcg’s scalar formula above.

Full disclosure: I have never used CVXQUAD. @awinick, who tried it, wrote in Adding Quantum Relative Entropy to CVX “I found that both the run time and memory usage meant that I could only consider trivial problems of interest.” But perhaps your problems of interest, if you’re still interested a year later, are smaller than his.

Update: I’ve now tried some optimization with quantum_rel_entrunder CVX 2.1 . As matrix dimension increases, it can take a long time to process even an expression having qunatum_rel_entr, let alone the solver solution time. And you can easily run out of memory.

I encountered an error in kron which I have not tried to diagnose when using it under CVX 3.0beta.