Definition:Accumulation Point/Sequence

Definition
Let $\left({S, \tau}\right)$ be a topological space.

Let $A \subseteq S$. Let $\left \langle {x_n} \right \rangle_{n \mathop \in \N}$ be an infinite sequence in $A$.

Let $x \in A$.

Suppose that:
 * $\forall U \in \tau: x \in U \implies \left\{{n \in \N: x_n \in U}\right\}$ is infinite

Then $x$ is an accumulation point of $\left \langle {x_n} \right \rangle$.