a Lebesgue measurable but non-Borel set


We give an example of a subset of the real numbers which is Lebesgue measurable, but not Borel measurable.

Let S consist of the set of all irrational real numbers with continued fractionMathworldPlanetmath of the form

x=a0+1a1+1a2+1

such that there exists an infiniteMathworldPlanetmathPlanetmath sequenceMathworldPlanetmath 0<i1<i2< where each aik divides aik+1. It can be shown that this set is Lebesgue measurable, but not Borel measurable.

In fact, it can be shown that S is an analytic setMathworldPlanetmath (http://planetmath.org/AnalyticSet2), meaning that it is the image of a continuous functionMathworldPlanetmathPlanetmath f:X for some Polish spaceMathworldPlanetmath X and, consequently, S is a universally measurable set.

This example is due to Lusin (1927).

Title a Lebesgue measurable but non-Borel set
Canonical name ALebesgueMeasurableButNonBorelSet
Date of creation 2013-03-22 18:37:01
Last modified on 2013-03-22 18:37:01
Owner gel (22282)
Last modified by gel (22282)
Numerical id 6
Author gel (22282)
Entry type Theorem
Classification msc 28A05
Classification msc 28A20