Dini's theorem

In the mathematical field of analysis, Dini's theorem says that if a monotone sequence of continuous functions converges pointwise on a compact space and if the limit function is also continuous, then the convergence is uniform.

Formal statement

If $X$ is a compact topological space, and $(f_n)_$ is a monotonically increasing sequence (meaning $f_n(x)\leq f_(x)$ for all $n\in\mathbb$ and $x\in X$) of continuous real-valued functions on $X$ which converges pointwise to a continuous function $f\colon X\to \mathbb$, then the convergence is uniform. The same conclusion holds if $(f_n)_$ is monotonically decreasing instead of increasing. The theorem is named after Ulisse Dini.According to , "his theoremis called Dini's theorem because Ulisse Dini (1845–1918) presented the original version of it in his book on the theory of functions of a real variable, published in Pisa in 1878".

This is one of the few situations in mathematics where pointwise convergence implies uniform convergence; the key is the greater control implied by the monotonicity. The limit function must be continuous, since a uniform limit of continuous functions is necessarily continuous. The continuity of the limit function cannot be inferred from the other hypothesis (consider $x^n$ in ,1/math>.)

Proof

Let $\varepsilon > 0$ be given. For each $n\in\mathbb$, let $g_n=f-f_n$, and let $E_n$ be the set of those $x\in X$ such that $g_n(x)<\varepsilon$. Each $g_n$ is continuous, and so each $E_n$ is open (because each $E_n$ is the preimage of the open set $(-\infty, \varepsilon)$ under $g_n$, a continuous function). Since $(f_n)_$ is monotonically increasing, $(g_n)_$ is monotonically decreasing, it follows that the sequence $E_n$ is ascending (i.e. $E_n\subset E_$ for all $n\in\mathbb$). Since $(f_n)_$ converges pointwise to $f$, it follows that the collection $(E_n)_$ is an open cover of $X$. By compactness, there is a finite subcover, and since $E_n$ are ascending the largest of these is a cover too. Thus we obtain that there is some positive integer $N$ such that $E_N=X$. That is, if $n>N$ and $x$ is a point in $X$, then $, f(x)-f_n(x), <\varepsilon$, as desired.

Notes

References

* Bartle, Robert G. and Sherbert Donald R.(2000) "Introduction to Real Analysis, Third Edition" Wiley. p 238. – Presents a proof using gauges.
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* Jost, Jürgen (2005) ''Postmodern Analysis, Third Edition,'' Springer. See Theorem 12.1 on page 157 for the monotone increasing case.
* Rudin, Walter R. (1976) ''Principles of Mathematical Analysis, Third Edition,'' McGraw–Hill. See Theorem 7.13 on page 150 for the monotone decreasing case.
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Theorems in real analysis
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