Existence of measures of maximal entropy for $C^r$ interval maps

Abstract

We show that a ${\mathcal{C}}^r$ $(r>1)$ map of the interval $f:[0,1]\to [0,1]$ with topological entropy larger than $\frac{\log \Vert f^{\prime }{\Vert }_{\mathrm{\infty }}}{r}$ admits at least one measure of maximal entropy. Moreover the number of measures of maximal entropy is finite. It is a sharp improvement of the 2006 paper of Buzzi and Ruette in the case of ${\mathcal{C}}^r$ maps and solves a conjecture of J. Buzzi stated in his 1995 thesis. The proof uses a variation of a theorem of isomorphism due to J. Buzzi between the interval map and the Markovian shift associated to the Buzzi-Hofbauer diagram.