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
Baron Augustin-Louis Cauchy (, ; ; 21 August 178923 May 1857) was a French mathematician, engineer, and physicist who made pioneering contributions to several branches of mathematics, including mathematical analysis and continuum mechanics. H ...
who published it in his textbook
Cours d'Analyse 1821.
Statement
A series
:
is convergent if and only if for every
there is a
natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called '' cardinal ...
''N'' such that
:
holds for all and all .
Explanation
The test works because the space
of real numbers and the space
of complex numbers (with the metric given by the absolute value) are both
complete. From here, the series is
convergent if and only if
In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false.
The connective is bi ...
the partial sum
:
is a
Cauchy sequence
In mathematics, a Cauchy sequence (; ), named after Augustin-Louis Cauchy, is a sequence whose elements become arbitrarily close to each other as the sequence progresses. More precisely, given any small positive distance, all but a finite numbe ...
.
Cauchy's convergence test can only be used in
complete metric space
In mathematical analysis, a metric space is called complete (or a Cauchy space) if every Cauchy sequence of points in has a limit that is also in .
Intuitively, a space is complete if there are no "points missing" from it (inside or at the bo ...
s (such as
and
), which are spaces where all Cauchy sequences converge. This is because we need only show that its elements become arbitrarily close to each other after a finite progression in the sequence to prove the series converges.
Proof
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
In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false.
The connective is bi ...
for every
there is a number ''N'', such that
m ≥ n ≥ N imply
:
Probably the most interesting part of
his theoremis 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".
References
{{reflist
Convergence tests