Rational approximation of Stieltjes functions by the Carathéodory-Fejér method

The Carathéodory-Fejér method provides a way for the construction of near best rational approximations. Gutknecht and Trefethen [7], [10] observed that in many cases the constructed functions yield very good estimates of the degree of rational approximation. We will Show that the correct asymptotic is obtained when the method is applied to Stieltjes functions. This fact is of interest in connection with Magnus' considerations of the 1/9-conjecture [9].