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In mathematics, a series is the sum of the terms of a sequence of numbers.## Examples of convergent and divergent series

## Convergence tests

## Conditional and absolute convergence

## Uniform convergence

## Cauchy convergence criterion

## References

## External links

Given a sequence $left\; \{\; a\_1,\; a\_2,\; a\_3,dots\; right\; \}$, the nth partial sum $S\_n$ is the sum of the first n terms of the sequence, that is,

- $S\_n\; =\; sum\_\{k=1\}^n\; a\_k.$

A series is convergent if the sequence of its partial sums $left\; \{\; S\_1,\; S\_2,\; S\_3,dots\; right\; \}$ converges. In more formal language, a series converges if there exists a limit $ell$ such that for any arbitrarily small positive number $varepsilon\; >\; 0$, there is a large integer $N$ such that for all $n\; ge\; N$,

- $left\; |\; S\_n\; -\; ell\; right\; vert\; le\; varepsilon.$

A sequence that is not convergent is said to be divergent.

- The reciprocals of powers of 2 produce a convergent series (so the set of powers of 2 is "small"):
- :$\{1\; over\; 1\}+\{1\; over\; 2\}+\{1\; over\; 4\}+\{1\; over\; 8\}+\{1\; over\; 16\}+\{1\; over\; 32\}+cdots\; =\; 2.$
- The reciprocals of positive integers produce a divergent series:
- :$\{1\; over\; 1\}+\{1\; over\; 2\}+\{1\; over\; 3\}+\{1\; over\; 4\}+\{1\; over\; 5\}+\{1\; over\; 6\}+cdots.$
- Alternating the signs of the reciprocals of positive integers produces a convergent series:
- :$\{1\; over\; 1\}-\{1\; over\; 2\}+\{1\; over\; 3\}-\{1\; over\; 4\}+\{1\; over\; 5\}-\{1\; over\; 6\}+cdots\; =\; ln\; 2.$
- The reciprocals of prime numbers produce a divergent series (so the set of primes is "large"):
- :$\{1\; over\; 2\}+\{1\; over\; 3\}+\{1\; over\; 5\}+\{1\; over\; 7\}+\{1\; over\; 11\}+\{1\; over\; 13\}+cdots\; .$
- The reciprocals of square numbers produce a convergent series (the Basel problem):
- :$\{1\; over\; 1\}+\{1\; over\; 4\}+\{1\; over\; 9\}+\{1\; over\; 16\}+\{1\; over\; 25\}+\{1\; over\; 36\}+cdots\; =\; \{pi^2\; over\; 6\}.$
- Alternating the signs of the reciprocals of positive odd numbers produces a convergent series:
- :$\{1\; over\; 1\}-\{1\; over\; 3\}+\{1\; over\; 5\}-\{1\; over\; 7\}+\{1\; over\; 9\}-\{1\; over\; 11\}+cdots\; =\; \{pi\; over\; 4\}.$

There are a number of methods of determining whether a series converges or diverges.

Comparison test. The terms of the sequence $left\; \{\; a\_n\; right\; \}$ are compared to those of another sequence $left\; \{\; b\_n\; right\; \}$. If,

for all n, $0\; le\; a\_n\; le\; b\_n$, and $sum\_\{n=1\}^infty\; b\_n$ converges, then so does $sum\_\{n=1\}^infty\; a\_n$.

However, if,

for all n, $0\; le\; b\_n\; le\; a\_n$, and $sum\_\{n=1\}^infty\; b\_n$ diverges, then so does $sum\_\{n=1\}^infty\; a\_n$.

Ratio test. Assume that for all n, $a\_n\; >\; 0$. Suppose that there exists $r$ such that

- $lim\_\{n\; to\; infty\}\; frac\{a\_\{n+1\}\}\{a\_n\}\; =\; r$.

If r < 1, then the series converges. If then the series diverges. If the ratio test is inconclusive, and the series may converge or diverge.

Root test or nth root test. Suppose that the terms of the sequence in question are non-negative, and that there exists r such that

- $lim\_\{n\; to\; infty\}\; sqrt[n]\{a\_n\}\; =\; r$

If r < 1, then the series converges. If then the series diverges. If the root test is inconclusive, and the series may converge or diverge.

The ratio test and the root test are both based on comparison with a geometric series, and as such they work in similar situations. In fact, if the ratio test works (meaning that the limit exists and is not equal to 1) then so does the root test; the converse, however, is not true. The root test is therefore more generally applicable, but as a practical matter the limit is often difficult to compute for commonly seen types of series.

Integral test. The series can be compared to an integral to establish convergence or divergence. Let $f(n)\; =\; a\_n$ be a positive and monotone decreasing function. If

- $int\_\{1\}^\{infty\}\; f(x),\; dx\; =\; lim\_\{t\; to\; infty\}\; int\_\{1\}^\{t\}\; f(x),\; dx\; <\; infty,$

then the series converges. But if the integral diverges, then the series does so as well.

Limit comparison test. If $left\; \{\; a\_n\; right\; \},\; left\; \{\; b\_n\; right\; \}\; >\; 0$, and the limit $lim\_\{n\; to\; infty\}\; frac\{a\_n\}\{b\_n\}$ exists and is not zero, then $sum\_\{n=1\}^infty\; a\_n$ converges if and only if $sum\_\{n=1\}^infty\; b\_n$ converges.

Alternating series test. Also known as the Leibniz criterion, the alternating series test states that for an alternating series of the form $sum\_\{n=1\}^infty\; a\_n\; (-1)^n$, if $left\; \{\; a\_n\; right\; \}$ is monotone decreasing, and has a limit of 0, then the series converges.

Cauchy condensation test. If $left\; \{\; a\_n\; right\; \}$ is a monotone decreasing sequence, then $sum\_\{n=1\}^infty\; a\_n$ converges if and only if $sum\_\{k=1\}^infty\; 2^k\; a\_\{2^k\}$ converges.

For any sequence $left\; \{\; a\_1,\; a\_2,\; a\_3,dots\; right\; \}$, $a\_n\; le\; left\; |\; a\_n\; right\; vert$ for all n. Therefore,

- $sum\_\{n=1\}^infty\; a\_n\; le\; sum\_\{n=1\}^infty\; left\; |\; a\_n\; right\; vert.$

This means that if $sum\_\{n=1\}^infty\; left\; |\; a\_n\; right\; vert$ converges, then $sum\_\{n=1\}^infty\; a\_n$ also converges (but not vice-versa).

If the series $sum\_\{n=1\}^infty\; left\; |\; a\_n\; right\; vert$ converges, then the series $sum\_\{n=1\}^infty\; a\_n$ is absolutely convergent. An absolutely convergent sequence is one in which the length of the line created by joining together all of the increments to the partial sum is finitely long. The power series of the exponential function is absolutely convergent everywhere.

If the series $sum\_\{n=1\}^infty\; a\_n$ converges but the series $sum\_\{n=1\}^infty\; left\; |\; a\_n\; right\; vert$ diverges, then the series $sum\_\{n=1\}^infty\; a\_n$ is conditionally convergent. The path formed by connecting the partial sums of a conditionally convergent series is infinitely long. The power series of the logarithm is conditionally convergent.

The Riemann series theorem states that if a series converges conditionally, it is possible to rearrange the terms of the series in such a way that the series converges to any value, or even diverges.

- Main article: uniform convergence.

Let $left\; \{\; f\_1,\; f\_2,\; f\_3,dots\; right\; \}$ be a sequence of functions. The series $sum\_\{n=1\}^infty\; f\_n$ is said to converge uniformly to f if the sequence $\{s\_n\}$ of partial sums defined by

- $s\_n(x)\; =\; sum\_\{k=1\}^n\; f\_k\; (x)$

converges uniformly to f.

There is an analogue of the comparison test for infinite series of functions called the Weierstrass M-test.

The Cauchy convergence criterion states that a series

- $sum\_\{n=1\}^infty\; a\_n$

- $left|\; sum\_\{k=m\}^n\; a\_k\; right|\; <\; varepsilon,$

- $lim\_\{n\; to\; infty\; atop\; mto\; infty\}\; sum\_\{k=n\}^\{n+m\}\; a\_k\; =\; 0.$

- Rudin, Walter (1976). Principles of Mathematical Analysis. McGrawHill.
- Spivak, Michael (1994). Calculus (3rd ed.). Houston, Texas: Publish or Perish, Inc. ISBN 0-914098-89-6.

- Interactive graphical simulation of series convergence
- Chase, Robert (2007). More plots on convergence
- Weisstein, Eric (2005). Riemann Series Theorem Retrieved May 16, 2005.

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Last updated on Thursday September 04, 2008 at 08:28:49 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Thursday September 04, 2008 at 08:28:49 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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