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Barb\u{a}lat's lemma (Theorem)
Lemma 1 (Barbalat)   Let $f \colon (0,\infty) \to {\mathbb{R}}$ be Riemann integrable and uniformly continuous then \begin{equation*} \lim_{t \to \infty} f(t) = 0 . \end{equation*}

Note that if $f$ is non-negative, then Riemann integrability is the same as being $L^1$ in the sense of Lebesgue, but if $f$ oscillates then the Lebesgue integral may not exist.

Further note that the uniform continuity is required to prevent sharp ``spikes'' that might prevent the limit from existing. For example suppose we add a spike of height 1 and area $2^{-n}$ at every integer. Then the function is continuous and $L^1$ (and thus Riemann integrable), but $f(t)$ would not have a limit at infinity.

Bibliography

1
Hartmut Logemann, Eugene P. Ryan. Asymptotic behaviour of nonlinear systems. The American Mathematical Monthly, 111(10):864-889, 2004.




"Barb\u{a}lat's lemma" is owned by jirka.
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Other names:  Barbalat's lemma

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proof of Barbalat's lemma (Proof) by ncrom
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Cross-references: infinity, continuous, function, integer, area, height, limit, uniform continuity, Lebesgue integral, Riemann, uniformly continuous, Riemann integrable

This is version 4 of Barb\u{a}lat's lemma, born on 2004-12-10, modified 2005-04-09.
Object id is 6552, canonical name is BarbualatsLemma.
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Classification:
AMS MSC26A06 (Real functions :: Functions of one variable :: One-variable calculus)

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