Error using cvx/pow_cvx (line 144) Disciplined convex programming error: Illegal operation: pow_p( {convex}, {-1} ) Error in cvx/inv_pos (line 5) y = pow_cvx( x, -1, 'pow_p' );

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help inv_pos

inv_pos Reciprocal of a positive quantity.
inv_pos(X) returns 1./X if X is positive, and +Inf otherwise.
X must be real.

 For matrices and N-D arrays, the function is applied to each element.

  Disciplined convex programming information:
      inv_pos is convex and nonincreasing; therefore, when used in CVX
      specifications, its argument must be concave (or affine).

The argument of inv_pos must be concave, but your argument is convex, because it is the output of another inv_pos.

Have you proven this problem is convex?