# The Big Three Pt. 6 - Closed Graph Theorem with Applications

(Before everything: elementary background of topology and vector spaces, Banach spaces, is assumed.)A surprising result of Banach spacesWe can define several relations between two norms. Suppose we have a topological vector space $X$ and two norms $\lVert \cdot \rVert_1$ and $\lVert \cdot \rVert_2$. One says $\lVert \cdot \rVert_1$ is weaker than $\lVert \cdot \rVert_2$ if there is $K>0$ such that $\lVert x \rVert_1 \leq K \lVert x \rVert_2$ for all $x \in X$. Two norms are equivalent if each i...