Cauchy's convergence test
The Cauchy convergence test is a method used to test infinite series for convergence. It relies on bounding sums of terms in the series. This convergence criterion is named after Augustin-Louis Cauchy who published it in his textbook Cours d'Analyse 1821.
- is convergent if and only if for every there is a natural number N such that
holds for all n > N and p ≥ 1.
The test works because the space R of real numbers and the space C of complex numbers (with the metric given by the absolute value) are both complete. Then the series is convergent if and only if the partial sum
is a Cauchy sequence.
A sequence of real or complex numbers is a Cauchy sequence if and only if converges (to some point a in R or C). The formal definition states that for every there is a number N, such that for all n, m > N holds
We will assume m > n and thus set p = m − n.
Showing that a sequence is a Cauchy sequence is useful since we do not need to know the limit of the sequence in question. Cauchy's convergence test can only be used in complete metric spaces (such as R and C), which are spaces where all Cauchy sequences converge. We need only show that its elements become arbitrarily close to each other after a finite progression in the sequence. There are computer applications of the Cauchy sequence, in which an iterative process may be set up to create such sequences.
We can use the results about convergence of the sequence of partial sums of the infinite series and apply them to the convergence of the infinite series itself. The Cauchy Criterion test is one such application. For any real sequence , the above results on convergence imply that the infinite series
converges if and only if for every there is a number N, such that
m ≥ n ≥ N imply
Probably the most interesting part of [this theorem] is that the Cauchy condition implies the existence of the limit: this is indeed related to the completeness of the real line. The Cauchy criterion can be generalized to a variety of situations, which can all be loosely summarized as "a vanishing oscillation condition is equivalent to convergence".
- cf. the answer to the question “Origin of Cauchy convergence test“ of the Q&A website “History of Science and Mathematics”
- Abbott, Stephen (2001). Understanding Analysis, p.63. Springer, New York. ISBN 9781441928665
- Wade, William (2010). An Introduction to Analysis. Upper Saddle River,NJ: Prentice Hall. p. 59. ISBN 9780132296380.
- Wade, William (2010). An Introduction to Analysis. Upper Saddle River, NJ: Prentice Hall. p. 188. ISBN 9780132296380.
- Encyclopedia of Mathematics. "Cauchy Criteria". European Mathematical Society. Retrieved 4 March 2014.