Before revision:  By a further development of the author's relaxation method, this paper provides a systemic approach to solving this new type of quantile optimization problems }  .  The optimal quantile is expressed via the solution of a free boundary problem for a  second-order nonlinear ordinary differential equation (ODE), which is similar to the Black-Scholes ODE for perpetual American options and has been well studied in literature theoretically and numerically .
After revision:  By a further development of the author's relaxation method, this paper provides a systemic approach to solving this new type of quantile optimization problems } . The optimal quantile is expressed via the solution of a free boundary problem for a second-order elliptic operator with mixed boundary conditions. Surprisingly, it is shown that every reasonable moral-hazard-free contract is optimal for infinitely many RDU maximisers with different utility functions and probability weighting functions .