Best approximations by rational functions with respect to the Hausdorff distance

Inverse theorems on the best approximations of plane sets in a Hausdorff metric by means of rational functions are cited. It is shown, among other things, that if $R_{n,r}(F,[a,b])=o(1/n)$, then there exists a set $P\subset[a,b]$ of complete measure over which $F$ constitutes a single-valued function.