# Ermakov convergence criterion

2020 Mathematics Subject Classification: Primary: 40A05 [MSN][ZBL]

A criterion for the convergence of a series $\sum_n f(n)$, where $f:[1, \infty[\to [0, \infty[$ is a monotone decreasing function, established by V.P. Ermakov in [Er].

Let $f(x)$ be a positive decreasing function for $x \ge 1$. If there is $\lambda< 1$ such that $\frac{e^x f(e^x)}{f(x)} < \lambda$ for sufficiently large $x$, then the series $\sum_n f(n)$ converges. If instead $\frac{e^x f(e^x)}{f(x)}\geq 1$ for all sufficiently large $x$, then the series diverges. In particular the convergence or divergence of the series can be decided if the limit $\lim_{x\to\infty} \frac{e^x f(e^x)}{f(x)}$ exists and differs from 1.

Ermakov's criterion can be derived from the integral test.

#### References

 [Br] T.J. Bromwich, "An introduction to the theory of infinite series" , Macmillan (1947) [Er] V.P. Ermakov, "A new criterion for convergence and divergence of infinite series of constant sign" , Kiev (1872) (In Russian)
How to Cite This Entry:
Ermakov convergence criterion. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Ermakov_convergence_criterion&oldid=35853
This article was adapted from an original article by L.D. Kudryavtsev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article