If you have never read one of Elias Stein’s books, you are missing out. “Harmonic Analysis: Real-Variables, Orthogonality, and Oscillatory” and “Introduction to Fourier Analysis on Euclidean Spaces” (co-authored by Guido Weiss) are not light bedtime reading, but they make the reader–or at least, me–get excited about Analysis. Moreover, I increasingly find the benefits of reading the masters when trying to learn something. Today, I want to share a result from Book 4 of the four-volume “Princeton Lectures in Analysis” geared towards undergraduates and beginning graduate students.

We will show that there exists a finitely-additive measure defined on all subsets of that is translation-invariant and agrees with the Lebesgue measure on the Lebesgue -algebra. Of course, such a measure has no hope of being countably additive (-additive) as we know from the existence of Vitali sets. Such a measure is called a Banach measure.

TTheorem 1here exists an extended-valued nonnegative function satisfying

(Finitely Additive) , if are disjoint;

(Agreement) if is Lebesgue measurable;

(Translation Invariance) for all and.

To prove Theorem 1, we will follow the functional analytic approach to measure and integration by first defining an integral as a linear functional on a suitable space of functions and then defining a measure by evaluating the integral of characteristic functions. Specifically, we use the Hahn-Banach theorem to extend the Lebesgue integral to the space of bounded (possibly nonmeasurable) functions on the -torus .

A quick remark on notation: we denote the power set and characteristic function of a set by and , respectively.

Theorem 2There exists a linear functional defined on all bounded functions on such that(Positive Semidefinite) , if ;

(Linearity) for all and ;

(Agreement) , if is Lebesgue measurable;

(Translation Invariance) , where , for all .

Proof: Let denote the real vector space of bounded functions . Let denote the subspace of Lebesgue measurable functions. Denote the functional on defined by Lebesgue integration by . To use the Hahn-Banach theorem, we need to find a suitable seminorm such that for all . The clever construction of is due to Polish mathematician Stefan Banach.

Let be a sequence in of length . Given , define

We define , where the infimum is taken over all finite sequencess of arbitrary length. The definition of should remind you of upper Darboux sums used in Riemann integration. It is clear that is well-defined, as is bounded.

I claim that is a sublinear functional on . Indeed, it is evident that is positively homgeneous. Given , we can find find sequences and of length and , respectively, such that and . Define a sequence of length . It is evident that . Since , where any and is a translate of , we see that

Taking the supremum of the LHS over all , we see that . A completely analogous argument shows that . Putting these inequalities together, we conclude that

Letting completes the proof of sublinearity.

I claim that for all . Indeed, by translation invariance and linearity,

for all finite sequences . Taking the infimum of the RHS over all such , we conclude that .

We apply the Hahn-Banach theorem to to obtain a linear functional bounded by . Properties (ii) and (iii) have already been proven. For (i), observe that if , then for all sequencess , whence . If , then applying this observation to shows that . To establish (iv), it suffices by symmetry to show that , where is fixed. Consider the set , for some integer . Then

Letting , we obtain that .

By defining by , we obtain the following corollary.

Corollary 3There exists a nonnegative set function such that

(Finite Additivity) , if are disjoint;

(Agreement) , if is Lebesgue measurable;

(Translation Invariance) for all .

We are now ready to prove Theorem 1. The plan is to partition into countably many disjoint cubes that can be translated to the unit cube by an element of . We first define locally on disjoint cubes by computing on the translate to . For an arbitrary set , we define as the “infinite sum” of these local quantities.

Proof of Theorem 1: Let denote the denote the translate of the unit cube by . The collection forms a countable partition of . For a subset , define

Observe that if , for some , then . Properties (i) and (ii) are evident from the corresponding properties of and the definition of .

To prove (iii), we will first show translation invariance for . Then by considering , it suffices to show translation invariance for .

For , observe that , whence

since is a bijection of . Now suppose that . For , set and define sets

We can write as the disjoint union . Observe that while , where . Using the finite additivity of and the -translation invariance of , we obtain

For , the existence of shows that there is no Banach-Tarski paradox in the real line, as the only nontrivial rigid motions are translations. It turns out that the Banach-Tarski paradox is also false in the plane. The interested reader in this result and more may consult Banach’s 1923 article “Sur le probleme de la mesure” and Banach’s 1924 article co-authored with Alfred Tarski. A more recent reference is Stan Wagon’s “The Banach-Tarski Paradox”.

[1] S. Banach, A. Tarski, Sur la decomposition des ensembles de points en parties respectivement congruentes, *Fund. math* **6** (1924), 118-148.

[2] S. Banach, Sur le probleme de la mesure, *Fund. math* **4** (1923), 7-33.

[3] E.M. Stein and R. Shakarchi, Functional Analysis: Introduction to Further Topics in Analysis (Princeton Lectures in Analysis) (Bk. 4), Princeton UP, 2011.

[4] S. Wagon, The Banach-Tarski Paradox, Cambridge UP, 1993.