Diophantine properties of fixed points of Minkowski question mark function
We consider irrational fixed points of the Minkowski question mark function $?(x)$, that is, irrational solutions of the equation $?(x)=x$. It is easy to see that there exist at least two such points. Although it is not known if there are other fixed points, we prove that the smallest and the greatest fixed points have irrationality measure exponent 2. We give more precise results about the approximation properties of these fixed points. Moreover, in the Appendix we introduce a condition from which it follows that there are only two irrational fixed points.