javicemarpe ha scritto:A set $A$ in a topologic space $X$ is closed if and only if it contains the limit of all its convergent sequences. Then, if $A$ is a subset of $X$, you have that its closure is equal to the "sequential closure", so, if "$\{x\}$" is the set of points of the sequential closure of $A$ which are not in the closure of $A$, you have that "$\{x\}$" does not contain any point, i.e., it is empty.
Ma perché dici che A è chiuso?