Duke Mathematical Journal

Gaps in √n mod 1 and ergodic theory

Noam D. Elkies and Curtis T. McMullen

Source: Duke Math. J. Volume 123, Number 1 (2004), 95-139.

Abstract

Cut the unit circle $S^1=\mathbb{R}/\mathbb{Z}$ at the points \{\sqrt{1}\}}, \{\sqrt{2}\},\ldots,\{\sqrt{N}\}, where $\{x\} = x \bmod 1$, and let $J_1, \ldots, J_N$ denote the complementary intervals, or \emph{gaps}, that remain. We show that, in contrast to the case of random points (whose gaps are exponentially distributed), the lengths $|J_i|/N$ are governed by an explicit piecewise real-analytic distribution $F(t) \,dt$ with phase transitions at $t=1/2$ and $t=2$.

The gap distribution is related to the probability $p(t)$ that a random unimodular lattice translate $\Lambda \subset \mathbb{R}^2$ meets a fixed triangle $S_t$ of area $t$; in fact, $p''(t) = -F(t)$. The proof uses ergodic theory on the universal elliptic curve

\[ E = \big(\SL_2(\mathbb{R}) \ltimes \mathbb{R}^2\big)/ \big(\SL_2(\mathbb{Z}) \ltimes \mathbb{Z}^2\big) \]

and Ratner's theorem on unipotent invariant measures.

Primary Subjects: 11J71, 22E40
Secondary Subjects: 37A17, 37A25

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Permanent link to this document: http://projecteuclid.org/euclid.dmj/1084479320
Digital Object Identifier: doi:10.1215/S0012-7094-04-12314-0
Mathematical Reviews number (MathSciNet): MR2060024

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