Tietze extension theorem

The Tietze Extension Theorem is a fundamental result in topology, particularly in the context of normal spaces. It states that if \( X \) is a normal topological space and \( A \) is a closed subset of \( X \), then any continuous function \( f: A \to \mathbb{R} \) can be extended to a continuous function \( F: X \to \mathbb{R} \).