# Doob–Meyer decomposition theorem

The Doob–Meyer decomposition theorem is a theorem in stochastic calculus stating the conditions under which a submartingale may be decomposed in a unique way as the sum of a martingale and an increasing predictable process. It is named for Joseph L. Doob and Paul-André Meyer.

## History

In 1953, Doob published the Doob decomposition theorem which gives a unique decomposition for certain discrete time martingales.[1] He conjectured a continuous time version of the theorem and in two publications in 1962 and 1963 Paul-André Meyer proved such a theorem, which became known as the Doob-Meyer decomposition.[2][3] In honor of Doob, Meyer used the term "class D" to refer to the class of supermartingales for which his unique decomposition theorem applied.[4]

## Class D Supermartingales

A càdlàg submartingale ${\displaystyle Z}$ is of Class D if ${\displaystyle Z_{0}=0}$ and the collection

${\displaystyle \{Z_{T}\mid {\text{T a finite valued stopping time}}\}}$

## The theorem

Let ${\displaystyle Z}$ be a cadlag submartingale of class D with ${\displaystyle Z_{0}=0}$. Then there exists a unique, increasing, predictable process ${\displaystyle A}$ with ${\displaystyle A_{0}=0}$ such that ${\displaystyle M_{t}=Z_{t}-A_{t}}$ is a uniformly integrable martingale.[5]

## Notes

1. Doob 1953
2. Meyer 1952
3. Meyer 1963
4. Protter 2005
5. Protter (2005)

## References

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