Fundamental Property of Norm on Bounded Linear Functional
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Theorem
Let $\Bbb F \in \set {\R, \C}$.
Let $\struct {X, \norm \cdot_X}$ be a normed vector space over $\Bbb F$.
Let $\struct {X^\ast, \norm \cdot_{X^\ast} }$ be the normed dual of $\struct {X, \norm \cdot_X}$.
Then for all $x \in X$ and $f \in X^\ast$:
- $\cmod {\map f x} \le \norm f_{X^\ast} \norm x_X$
Proof
Let $x \in X$ and $f \in X^\ast$.
If $x = 0$, the claim is trivial, since by the linearity of $f$:
- $\cmod {\map f x} = \cmod {\map f 0} = 0$
Let $x \ne 0$.
Recall by Definition 3 of Norm of Bounded Linear Functional:
- $\norm f_{X^\ast} = \sup \set {\dfrac {\cmod {\map f x} } {\norm x_X} : \forall x \in X, x \ne 0}$
Thus, by $\paren 1$ of Definition of Supremum:
- $\cmod {\map f x} = \dfrac {\cmod {\map f x} }{\norm x_X} {\norm x_X} \le \norm f_{X^\ast} \norm x_X$
$\blacksquare$