Dual norm

In functional analysis, the dual norm is a measure of the "size" of continuous linear functionals.

Let ${\displaystyle X}$ and ${\displaystyle Y}$ be topological vector spaces, and ${\displaystyle L(X,Y)}$ be the collection of all bounded linear mappings (or operators) of ${\displaystyle X}$ into ${\displaystyle Y}$. In the case where ${\displaystyle X}$ and ${\displaystyle Y}$ are normed vector spaces, ${\displaystyle L(X,Y)}$ can be normed in a natural way.

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