# Cauchy test

The Cauchy criterion for the convergence of a series: Given a series $\sum_{n=1}^{\infty}u_n$ with non-negative real terms, if there exists a number $q$, $0\leq q<1$, such that, for all sufficiently large $n$, one has the inequality $(u_n)^{1/n}\leq q$, which is equivalent to the condition , then the series is convergent. Conversely, if for all sufficiently large one has the inequality , or even the weaker condition: There exists a subsequence , with terms satisfying the inequality , then the series is divergent.

In particular, if exists and is , then the series is convergent; if it is , then the series is divergent. This was proved by A.L. Cauchy . In the case of a series with terms of arbitrary sign, the condition implies that the series is divergent; if , the series is absolutely convergent.

The integral Cauchy test, or the Cauchy–MacLaurin integral criterion: Given a series with non-negative real terms, if there exists a non-increasing non-negative function , defined for , such that , then the series is convergent if and only if the integral is convergent. This test was first presented in a geometrical form by C. MacLaurin [2], and later rediscovered by A.L. Cauchy [3].

#### References

 [1] A.L. Cauchy, "Analyse algébrique" , Gauthier-Villars (1821) pp. 132–135 (German translation: Springer, 1885) [2] C. MacLaurin, "Treatise of fluxions" , 1 , Edinburgh (1742) pp. 289–290 [3] A.L. Cauchy, "Sur la convergence des séries" , Oeuvres complètes Ser. 2 , 7 , Gauthier-Villars (1889) pp. 267–279 [4] S.M. Nikol'skii, "A course of mathematical analysis" , 1 , MIR (1977) (Translated from Russian)