A Resolution of the de Bruijn--Erdős Consecutive-Gap Problem

Abstract

Let $(x_n)_{n\geq1}$ be a sequence of distinct points on the unit circle. An $r$-span is the total length of $r$ consecutive gaps determined by the inserted points. Write $M_n^{(r)}$ and $m_n^{(r)}$ for the largest and smallest $r$-spans after the first $n$ insertions. We prove that there is an absolute constant $c>0$ such that, for every sufficiently large $r$, \[ \limsup_{n\to\infty}\bigl(nM_n^{(r)}-r\bigr) \geq c\sqrt{\log r}, \qquad \limsup_{n\to\infty}\bigl(r-nm_n^{(r)}\bigr) \geq c\sqrt{\log r}, \] and \[ \limsup_{n\to\infty}\frac{M_n^{(r)}}{m_n^{(r)}} \geq 1+\frac{\log r}{100r}. \] Thus all three asymptotic conjectures made by de Bruijn and Erdős in 1949 are resolved. The ratio bound matches the upper bound of Clément and Steinerberger up to an absolute constant and answers a question of Brethouwer. The proofs compare interval counts at nearby times. Pointwise control leads to a one-dimensional sequence-discrepancy argument for the ratio, while averaged control and Halász's planar $L^1$ discrepancy theorem give the two one-sided conclusions.