Null set

 ( Measure Theory ) 

In mathematical analysis, a null set is a Lebesgue measurable set of real numbers that has Lebesgue measure zero. This can be characterized as a set that can be covered by a countable union of intervals of arbitrarily small total length.

The notion of null set should not be confused with the empty set as defined in set theory. Although the empty set has Lebesgue measure zero, there are also non-empty sets which are null. For example, any non-empty countable set of real numbers has Lebesgue measure zero and therefore is null.

More generally, on a given measure space $M\; =\; (X,\; \backslash Sigma,\; \backslash mu)$ a null set is a set $S\; \backslash in\; \backslash Sigma$ such that $\backslash mu(S)\; =\; 0.$

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Examples Every finite or countably infinite subset of the real numbers is a null set. For example, the set of natural numbers , the set of rational numbers and the set of algebraic numbers are all countably infinite and therefore are null sets when considered as subsets of the real numbers.

The Cantor set is an example of an uncountable null set. It is uncountable because it contains all real numbers between 0 and 1 whose ternary expansion can be written using only 0s and 2s (see Cantor's diagonal argument), and it is null because it is constructed by beginning with the closed interval of real numbers from 0 to 1 and iteratively removing a third of the previous set, thereby multiplying the length by 2/3 with every step.

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Definition for Lebesgue measure The Lebesgue measure is the standard way of assigning a length, area or volume to subsets of Euclidean space.

A subset $N$ of the real line $\backslash Reals$ has null Lebesgue measure and is considered to be a null set (also known as a set of zero-content) in $\backslash Reals$ if and only if:

(In terminology of mathematical analysis, this definition requires that there be a sequence of of $A$ for which the limit of the lengths of the covers is zero.)

This condition can be generalised to $\backslash Reals^n,$ using $n$-cubes instead of intervals. In fact, the idea can be made to make sense on any manifold, even if there is no Lebesgue measure there.

For instance:

• With respect to $\backslash Reals^n,$ all singleton sets are null, and therefore all are null. In particular, the set $\backslash Q$ of is a null set, despite being dense in $\backslash Reals.$
• The standard construction of the Cantor set is an example of a null uncountable set in $\backslash Reals;$ however other constructions are possible which assign the Cantor set any measure whatsoever.
• All the subsets of $\backslash Reals^n$ whose dimension is smaller than $n$ have null Lebesgue measure in $\backslash Reals^n.$ For instance straight lines or circles are null sets in $\backslash Reals^2.$
• Sard's lemma: the set of critical values of a smooth function has measure zero.

If $\backslash lambda$ is Lebesgue measure for $\backslash Reals$ and π is Lebesgue measure for $\backslash Reals^2$, then the product measure $\backslash lambda\; \backslash times\; \backslash lambda\; =\; \backslash pi.$ In terms of null sets, the following equivalence has been styled a Fubini's theorem:

• For $A\; \backslash subset\; \backslash Reals^2$ and $A\_x\; =\; \backslash \{y\; :\; (x\; ,\; y)\; \backslash isin\; A\backslash \},$ $$\backslash pi(A)\; =\; 0\; \backslash iff\; \backslash lambda\; \backslash left(\backslash left\backslash \{x\; :\; \backslash lambda\backslash left(A\_x\backslash right)\; >\; 0\backslash right\backslash \}\backslash right)\; =\; 0.$$

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Measure-theoretic properties Let $(X,\backslash Sigma,\backslash mu)$ be a measure space. We have:

• $\backslash mu(\backslash varnothing)\; =\; 0$ (by definition of $\backslash mu$).
• Any countable union of null sets is itself a null set (by countable subadditivity of $\backslash mu$).
• Any (measurable) subset of a null set is itself a null set (by monotonicity of $\backslash mu$).

Together, these facts show that the null sets of $(X,\backslash Sigma,\backslash mu)$ form a Sigma-ideal of the sigma-algebra $\backslash Sigma$. Accordingly, null sets may be interpreted as , yielding a measure-theoretic notion of "almost everywhere".

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Uses Null sets play a key role in the definition of the Lebesgue integral: if functions $f$ and $g$ are equal except on a null set, then $f$ is integrable if and only if $g$ is, and their integrals are equal. This motivates the formal definition of Lp space as sets of equivalence classes of functions which differ only on null sets.

A measure in which all subsets of null sets are measurable is complete measure. Any non-complete measure can be completed to form a complete measure by asserting that subsets of null sets have measure zero. Lebesgue measure is an example of a complete measure; in some constructions, it is defined as the completion of a non-complete Borel measure.

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A subset of the Cantor set which is not Borel measurable The Borel measure is not complete. One simple construction is to start with the standard Cantor set $K,$ which is closed hence Borel measurable, and which has measure zero, and to find a subset $F$ of $K$ which is not Borel measurable. (Since the Lebesgue measure is complete, this $F$ is of course Lebesgue measurable.)

First, we have to know that every set of positive measure contains a nonmeasurable subset. Let $f$ be the Cantor function, a continuous function which is locally constant on $K^c,$ and monotonically increasing on $0,,$ with $f(0)\; =\; 0$ and $f(1)\; =\; 1.$ Obviously, $f(K^c)$ is countable, since it contains one point per component of $K^c.$ Hence $f(K^c)$ has measure zero, so $f(K)$ has measure one. We need a strictly monotonic function, so consider $g(x)\; =\; f(x)\; +\; x.$ Since $g$ is strictly monotonic and continuous, it is a homeomorphism. Furthermore, $g(K)$ has measure one. Let $E\; \backslash subseteq\; g(K)$ be non-measurable, and let $F\; =\; g^\{-1\}(E).$ Because $g$ is injective, we have that $F\; \backslash subseteq\; K,$ and so $F$ is a null set. However, if it were Borel measurable, then $f(F)$ would also be Borel measurable (here we use the fact that the preimage of a Borel set by a continuous function is measurable; $g(F)\; =\; (g^\{-1\})^\{-1\}(F)$ is the preimage of $F$ through the continuous function $h\; =\; g^\{-1\}$). Therefore $F$ is a null, but non-Borel measurable set.

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Haar null In a separable space Banach space $(X,\; \backslash |\backslash cdot\backslash |),$ addition moves any subset $A\; \backslash subseteq\; X$ to the translates $A\; +\; x$ for any $x\; \backslash in\; X.$ When there is a probability measure on the σ-algebra of of $X,$ such that for all $x,$ $\backslash mu(A\; +\; x)\; =\; 0,$ then $A$ is a Haar null set.

The term refers to the null invariance of the measures of translates, associating it with the complete invariance found with Haar measure.

Some algebraic properties of topological groups have been related to the size of subsets and Haar null sets. Haar null sets have been used in to show that when is not a meagre set then $A^\{-1\}\; A$ contains an open neighborhood of the identity element. This property is named for Hugo Steinhaus since it is the conclusion of the Steinhaus theorem.

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Further reading

• (2025). 9781852337810, Springer. ISBN 9781852337810
• Frank Jones (1993). 9780867202038, Jones & Bartlett. ISBN 9780867202038
• John C. Oxtoby (1971). 9780387053493, Springer-Verlag. ISBN 9780387053493

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