Reciprocal of a variable:1/x

Nonconvex
2

The argument of inv_pos needs to be concave (affine is a special case of concave). Actually, not only is s*s' not concave, it’s an n by n matrix, not a scalar. Did you mean Psii_ini=s'*s; ?

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).