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Formally, a Cauchy sequence is a sequence
in a metric space (M, d) such that for every positive real number r > 0, there is an integer N such that for all integers m and n greater than N the distance
is less than r. Roughly speaking, the terms of the sequence are getting closer and closer together in a way that suggests that the sequence ought to have a limit in M. Nonetheless, this may not be the case.
A metric space X in which every Cauchy sequence has a limit (in X) is called complete. The real numbers are complete, and the standard construction of the real numbers involves Cauchy sequences of rational numbers. The rational numbers themselves are not complete: a sequence of rational numbers can have the square root of two as its limit, for example. See Complete space for an example of a Cauchy sequence of rational numbers that does not have a rational limit.
Every convergent sequence is a Cauchy sequence, and every Cauchy sequence is bounded. If is a uniformly continuous map between the metric spaces M and N and (xn) is a Cauchy sequence in M, then is a Cauchy sequence in N. If and are two Cauchy sequences in the rational, real or complex numbers, then the sum and the product are also Cauchy sequences.