Positive measure and level sets of the Takagi function

Abstract

Let $T$ be the classical Takagi function, and $L(y):=\{x\in [0,1]:T(x)=y\}$ be its level set at height $y$. For each positive integer $m$, let $S_m$ be the set of $y$ for which $L(y)$ has exactly $m$ points. We prove that $S_{2n}$ has positive Lebesgue measure for every positive integer $n$, confirming a conjecture of Allaart.