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In topology, a point p is said to be an accumulation point of a set X if every neighborhood of p contains points of X distinct from p. (Note that p need not be inX.)

For example, on the real line with the usual topology, 0 is an accumulation point of the open interval bounded by 0 and 1, because any neighborhood of 0 contains points (infinitely many, in fact) lying between 0 and 1.

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