Equivalence of Definitions of Norm of Linear Functional

Theorem
Let $H$ be a Hilbert space, and let $L$ be a bounded linear functional on $H$.

Define the following norms of $L$:


 * $(1): \quad \norm L_1 = \sup \set {\size {L h}: \norm h \le 1}$
 * $(2): \quad \norm L_2 = \sup \set {\size {L h}: \norm h = 1}$
 * $(3): \quad \norm L_3 = \sup \set {\dfrac {\size {L h} } {\norm h}: h \in H \setminus \set {\mathbf 0} }$
 * $(4): \quad \norm L_4 = \inf \set {c > 0: \forall h \in H: \size {L h} \le c \norm h}$

Then:
 * $\norm L_1 = \norm L_2 = \norm L_3 = \norm L_4$

Proof
We have:
 * $\set {h \in H : \norm h = 1} \subseteq \set {h \in H : \norm h \le 1} \subseteq H$

So it follows from the definition of the supremum that
 * $\norm L_2 \le \norm L_1 \le \norm L_3$

Next we show that $\norm L_2 = \norm L_3$:

Therefore
 * $\norm L_1 = \norm L_2 = \norm L_3$

Moreover, if $\size {L h} \le c \norm h$ for all $h \in H \setminus \set {\mathbf 0}$, then we have:
 * $\norm L_3 = \sup \set {\dfrac {\size {L h} } {\norm h}: h \in H \setminus \set {\mathbf 0} } \le c$

Taking the infimum over all such $c$ this reads:
 * $\norm L_3 \le \norm L_4$

$c_0 := \norm L_4 > \norm L_3$.

Then by the definitions of these two norms, this means that there exists $\epsilon > 0$ such that for every $h \in H \setminus \set {\mathbf 0}$:
 * $\dfrac {\size {L h} } {\norm h} + \epsilon \le c_0$

But this in turn implies that for every $h \in H \setminus \set {\mathbf 0}$:
 * $\size {L h} \le c_0 \norm h - \epsilon \norm h = \paren {c_0 - \epsilon} \norm h$

This contradicts the fact that $c_0$ is the least such number satisfying this inequality.

Therefore:
 * $\norm L_3 = \norm L_4$

and the proof is complete.