[:second-countable:]
• A topological space $X$ is called *second-countable* iff it has a topological basis which is countable
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• Examples:
• $\mathbb R$ -- take the collection of all open intervals with rational endpoints; this is a countable basis
• Similarly, $\mathbb R^n$ and even $\mathbb R^\omega$
• Easily stronger than being first-countable