Question

When do you say that a topological space is second countable ?Given an example of a second countable topological space

Answer 1

A second-countable space is a topological space whose topology has a countable base. For example, the class of open intervals $(a, b)$ with rational endpoints $a,b\in\mathbb Q$ is countable and is a base for the usual topology on the real line $\R$ . Thus $\R$ is a second countable space.