In Calculus what is an accumulation point?

Answer 1

An accumulation point, or limit point, for a set S is a point x (not necessarily in S) such that any open set containing x also contains a point (distinct from x) that's in S. More intuitively, it means that by choosing points in S, we can get as close as we want to x without actually reaching it. For example, consider the set S={1,1/2,1/3,1/4,...} (in the real numbers). 0 is an accumulation point for S, because any open set containing 0 would have to contain all between 0 and some ε>0, which would include a point (actually, an infinite amount of points) in S. But 1/5, for example, is not an accumulation point for S, because we can take the open interval (11/60,9/40) which doesn't contain any points in S other than 1/5. Not all sets have an accumulation point. For example, any set of a finite amount of real numbers can't have an accumulation point. Another example of a set without an accumulation point is the integers (as a subset of the real numbers). However, over the real numbers, any bounded infinite set has an accumulation point. In a general topological space, any infinite subset of a compact set has an accumulation point.