In mathematics, **lifting theory** was first introduced by John von Neumann in a pioneering paper from 1931, in which he answered a question raised by Alfréd Haar.^{[1]} The theory was further developed by Dorothy Maharam (1958)^{[2]} and by Alexandra Ionescu Tulcea and Cassius Ionescu Tulcea (1961).^{[3]} Lifting theory was motivated to a large extent by its striking applications. Its development up to 1969 was described in a monograph of the Ionescu Tulceas.^{[4]} Lifting theory continued to develop since then, yielding new results and applications.

A **lifting** on a measure space is a linear and multiplicative operator
which is a right inverse of the quotient map

where is the seminormed L^{p} space of measurable functions and is its usual normed quotient. In other words, a lifting picks from every equivalence class of bounded measurable functions modulo negligible functions a representative— which is henceforth written or or simply — in such a way that and for all and all

Liftings are used to produce disintegrations of measures, for instance conditional probability distributions given continuous random variables, and fibrations of Lebesgue measure on the level sets of a function.

Theorem.Suppose is complete.^{[5]}Then admits a lifting if and only if there exists a collection of mutually disjoint integrable sets in whose union is In particular, if is the completion of aσ-finite^{[6]}measure or of an inner regular Borel measure on a locally compact space, then admits a lifting.

The proof consists in extending a lifting to ever larger sub-*σ*-algebras, applying Doob's martingale convergence theorem if one encounters a countable chain in the process.

Suppose is complete and is equipped with a completely regular Hausdorff topology such that the union of any collection of negligible open sets is again negligible – this is the case if is *σ*-finite or comes from a Radon measure. Then the *support* of can be defined as the complement of the largest negligible open subset, and the collection of bounded continuous functions belongs to

A **strong lifting** for is a lifting
such that on for all in This is the same as requiring that^{[7]} for all open sets in

Theorem.If isσ-finite and complete and has a countable basis then admits a strong lifting.

**Proof.** Let be a lifting for and a countable basis for For any point in the negligible set
let be any character^{[8]} on that extends the character of Then for in and in define:
is the desired strong lifting.

Suppose and are *σ*-finite measure spaces ( positive) and is a measurable map. A **disintegration of along with respect to ** is a slew of positive *σ*-additive measures on such that

- is carried by the fiber of over , i.e. and for almost all
- for every -integrable function in the sense that, for -almost all in is -integrable, the function is -integrable, and the displayed equality holds.

Disintegrations exist in various circumstances, the proofs varying but almost all using strong liftings. Here is a rather general result. Its short proof gives the general flavor.

Theorem.Suppose is a Polish space^{[9]}and a separable Hausdorff space, both equipped with their Borelσ-algebras. Let be aσ-finite Borel measure on and a measurable map. Then there exists a σ-finite Borel measure on and a disintegration (*). If is finite, can be taken to be the pushforward^{[10]}and then the are probabilities.

**Proof.** Because of the polish nature of there is a sequence of compact subsets of that are mutually disjoint, whose union has negligible complement, and on which is continuous. This observation reduces the problem to the case that both and are compact and is continuous, and Complete under and fix a strong lifting for Given a bounded -measurable function let denote its conditional expectation under that is, the Radon-Nikodym derivative of^{[11]} with respect to Then set, for every in To show that this defines a disintegration is a matter of bookkeeping and a suitable Fubini theorem. To see how the strongness of the lifting enters, note that
and take the infimum over all positive in with it becomes apparent that the support of lies in the fiber over

**^**von Neumann, John (1931). "Algebraische Repräsentanten der Funktionen "bis auf eine Menge vom Maße Null"".*Journal für die reine und angewandte Mathematik*(in German).**1931**(165): 109–115. doi:10.1515/crll.1931.165.109. MR 1581278.**^**Maharam, Dorothy (1958). "On a theorem of von Neumann".*Proceedings of the American Mathematical Society*.**9**(6): 987–994. doi:10.2307/2033342. JSTOR 2033342. MR 0105479.**^**Ionescu Tulcea, Alexandra; Ionescu Tulcea, Cassius (1961). "On the lifting property. I."*Journal of Mathematical Analysis and Applications*.**3**(3): 537–546. doi:10.1016/0022-247X(61)90075-0. MR 0150256.**^**Ionescu Tulcea, Alexandra; Ionescu Tulcea, Cassius (1969).*Topics in the theory of lifting*. Ergebnisse der Mathematik und ihrer Grenzgebiete. Vol. 48. New York: Springer-Verlag. MR 0276438. OCLC 851370324.**^**A subset is locally negligible if it intersects every integrable set in in a subset of a negligible set of is*complete*if every locally negligible set is negligible and belongs to**^**i.e., there exists a countable collection of integrable sets – sets of finite measure in – that covers the underlying set**^**are identified with their indicator functions.**^**A*character*on a unital algebra is a multiplicative linear functional with values in the coefficient field that maps the unit to 1.**^**A separable space is*Polish*if its topology comes from a complete metric. In the present situation it would be sufficient to require that is*Suslin*, that is, is the continuous Hausdorff image of a Polish space.**^**The*pushforward*of under also called the image of under and denoted is the measure on defined by for in .**^**is the measure that has density with respect to

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Types of Measures | - Atomic
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Main results | - Carathéodory's extension theorem
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