I have a new question, the expression is like (a^2+b^2)^n ,where a and b are variables, n is a constant and n>1. can it be transformed into something like norm expression , I guess?

`pow_pos(norm([a b]),2*n)`

You would benefit from a careful reading of chapter 9 of the CVX Usersâ€™ Guide http://cvxr.com/cvx/doc/funcref.html#

Wow, I donâ€™t know the pow_pos can perform convex^n,(n>1).

You need to look at help, as opposed to Usersâ€™ Guide, to see full usage rules.

help pow_pos

pow_pos Power of positive part.

pow_pos(X,P) = POS(X).^P = MAX(X,0).^P.

Both P and X must be real, and P must be greater than or equal to 1.`Disciplined convex programming information: pow_pos(X,P) is convex and nondecreasing in X; so when used in CVX expressions, X must be convex. P must be constant, real, and greater than or equal to 1.`