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Kummer criterion

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2010 Mathematics Subject Classification: Primary: 40A05 [MSN][ZBL]

A general convergence criterion for series with positive terms, proposed by E. Kummer. Let \begin{equation}\tag{1} \sum_n a_n \end{equation} be a series of positive numbers and $\{c_n\}$ a sequence of positive numbers. If there are $\delta >0$ and $N$ such that \[ K_n := c_n \frac{a_n}{a_{n+1}} - c_{n+1} \geq \delta \qquad \forall n\geq N\, , \] then (1) converges. If the series $\sum_n (c_n)^{-1}$ diverges and there is $N$ such that $K_n \leq 0$ for all $n\geq N$, then (1) diverges.

An obvious corollary is that, when the limit \[ K := \lim_{n\to \infty} K_n \] exists we have:

  • if $K>0$ (1) converges
  • if $K<0$ and $\sum_n (c_n)^{-1}$ diverges, then (1) diverges.


References

[Fi] G.M. Fichtenholz, "Differential und Integralrechnung" , 1 , Deutsch. Verlag Wissenschaft. (1964)
[Ra] E.D. Rainville, "Infinite series" , Macmillan (1967)
How to Cite This Entry:
Kummer criterion. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Kummer_criterion&oldid=30925
This article was adapted from an original article by E.G. Sobolevskaya (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article