# Birkhoff ergodic theorem

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2010 Mathematics Subject Classification: Primary: 37A30 Secondary: 37A0537A10 [MSN][ZBL]

One of the most important theorems in ergodic theory. For an endomorphism $T$ of a $\sigma$-finite measure space $(X,\Sigma,\mu)$, Birkhoff’s ergodic theorem states that for any function $f \in {L^{1}}(X,\Sigma,\mu)$, the limit $$\overline{f}(x) \stackrel{\text{df}}{=} \lim_{n \to \infty} \frac{1}{n} \sum_{k = 0}^{n - 1} f \! \left( {T^{k}}(x) \right)$$ (the time average or the average along a trajectory) exists almost everywhere (for almost all $x \in X$). Moreover, $\overline{f} \in {L^{1}}(X,\Sigma,\mu)$, and if $\mu(X) < \infty$, then $$\int_{X} f ~ \mathrm{d}{\mu} = \int_{X} \overline{f} ~ \mathrm{d}{\mu}.$$

For a measurable flow $(T_{t})_{t \geq 0}$ in a $\sigma$-finite measure space $(X,\Sigma,\mu)$, Birkhoff’s ergodic theorem states that for any function $f \in {L^{1}}(X,\Sigma,\mu)$, the limit $$\overline{f}(x) \stackrel{\text{df}}{=} \lim_{t \to \infty} \frac{1}{t} \int_{0}^{t} f({T_{t}}(x)) ~ \mathrm{d}{t}$$ exists almost everywhere, with the same properties as $f$.

Birkhoff’s theorem was stated and proved by G.D. Birkhoff [B]. It was then modified and generalized in various ways (there are theorems that contain, in addition to Birkhoff’s theorem, also a number of statements of a somewhat different kind, which are known in probability theory as ergodic theorems (cf. Ergodic theorem); there also exist ergodic theorems for more general semi-groups of transformations [KSS]). Birkhoff’s ergodic theorem and its generalizations are known as individual ergodic theorems, since they deal with the existence of averages along almost each individual trajectory, as distinct from statistical ergodic theorems — the von Neumann ergodic theorem and its generalizations. (In non-Soviet literature, the term “pointwise ergodic theorem” is often used to stress the fact that the averages are almost-everywhere convergent.)

#### References

 [B] G.D. Birkhoff, “Proof of the ergodic theorem”, Proc. Nat. Acad. Sci. USA, 17 (1931), pp. 656–660. Zbl 0003.25602 Zbl 57.1011.02 [KSS] A.B. Katok, Ya.G. Sinai, A.M. Stepin, “Theory of dynamical systems and general transformation groups with invariant measure”, J. Soviet Math., 7: 6 (1977), pp. 974–1065; Itogi Nauk. i Tekhn. Mat. Analiz, 13 (1975), pp. 129–262. MR0584389 Zbl 0399.28011