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In mathematical analysis, the **alternating series test** is the method used to show that an alternating series is convergent when its terms (1) decrease in absolute value, and (2) approach zero in the limit.
The test was used by Gottfried Leibniz and is sometimes known as **Leibniz's test**, **Leibniz's rule**, or the **Leibniz criterion**.

A series of the form

where either all *a*_{n} are positive or all *a*_{n} are negative, is called an alternating series.

The **alternating series test** guarantees that an alternating series converges if the following two conditions are met:

- decreases monotonically
^{[1]}, i.e., , and

Moreover, let *L* denote the sum of the series, then the partial sum

approximates *L* with error bounded by the next omitted term:

Suppose we are given a series of the form , where and for all natural numbers *n*. (The case follows by taking the negative.)^{[1]}

We will prove that both the partial sums with odd number of terms, and with even number of terms, converge to the same number *L*. Thus the usual partial sum also converges to *L*.

The odd partial sums decrease monotonically:

while the even partial sums increase monotonically:

both because *a*_{n} decreases monotonically with *n*.

Moreover, since *a*_{n} are positive, . Thus we can collect these facts to form the following suggestive inequality:

Now, note that *a*_{1} − *a*_{2} is a lower bound of the monotonically decreasing sequence *S*_{2m+1}, the monotone convergence theorem then implies that this sequence converges as *m* approaches infinity. Similarly, the sequence of even partial sum converges too.

Finally, they must converge to the same number because

Call the limit *L*, then the monotone convergence theorem also tells us extra information that

for any *m*. This means the partial sums of an alternating series also "alternates" above and below the final limit. More precisely, when there is an odd (even) number of terms, i.e. the last term is a plus (minus) term, then the partial sum is above (below) the final limit.

This understanding leads immediately to an error bound of partial sums, shown below.

We would like to show by splitting into two cases.

When k = 2m+1, i.e. odd, then

When k = 2m, i.e. even, then

as desired.

Both cases rely essentially on the last inequality derived in the previous proof.

For an alternative proof using Cauchy's convergence test, see Alternating series.

For a generalization, see Dirichlet's test.

The alternating harmonic series

meets both conditions for the alternating series test and converges.

All of the conditions in the test, namely convergence to zero and monotonicity, should be met in order for the conclusion to be true. For example, take the series

The signs are alternating and the terms tend to zero. However, monotonicity is not present and we cannot apply the test. Actually the series is divergent. Indeed, for the partial sum we have which is twice the partial sum of the harmonic series, which is divergent. Hence the original series is divergent.

**^**In practice, the first few terms may increase. What is important is that for all after some point,^{[2]}because the first finite amount of terms would not change a series' convergence/divergence.

- Konrad Knopp (1956)
*Infinite Sequences and Series*, § 3.4, Dover Publications ISBN 0-486-60153-6 - Konrad Knopp (1990)
*Theory and Application of Infinite Series*, § 15, Dover Publications ISBN 0-486-66165-2 - James Stewart, Daniel Clegg, Saleem Watson (2016)
*Single Variable Calculus: Early Transcendentals (Instructor's Edition) 9E*, Cengage ISBN 978-0-357-02228-9 - E. T. Whittaker & G. N. Watson (1963)
*A Course in Modern Analysis*, 4th edition, §2.3, Cambridge University Press ISBN 0-521-58807-3

- Weisstein, Eric W. "Leibniz Criterion".
*MathWorld*. - Jeff Cruzan. "Alternating series"