**Open Cover:**

Let be a topological space. A collection of open subsets of is said to be open cover for if .

A sub-collection of an open cover which is itself an open cover is called a sub-cover.

**Lindelof Space:**

A topological space is said to be a Lindelof space if every open cover of has a countable sub-cover.

**Theorems:**

• A closed sub-space of Lindelof space is Lindelof.

• Every second countable space is Lindelof space.

**Lindelof Theorem:**

Let be a second countable space. If a non-empty open set in is represented as the union of a collection of open sets, then can be represented as a countable union of .

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