Sum of Bounded Linear Transformations is Bounded Linear Transformation
Theorem
Let $\mathbb F \in \set {\R, \C}$.
Let $\struct {\HH, \innerprod \cdot \cdot_\HH}$ and $\struct {\KK, \innerprod \cdot \cdot_\KK}$ be Hilbert spaces over $\mathbb F$.
Let $A, B : \HH \to \KK$ be bounded linear transformations.
Let $\norm \cdot$ be the norm on the space of bounded linear transformations.
Then:
- $A + B$ is a bounded linear transformation
with:
- $\norm {A + B} \le \norm A + \norm B$
Proof
From Addition of Linear Transformations, we have that:
- $A + B$ is a linear transformation.
It remains to show that $A + B$ is bounded.
Let $\norm \cdot_\HH$ be the inner product norm on $\HH$.
Let $\norm \cdot_\KK$ be the inner product norm on $\KK$.
Since $A$ is a bounded linear transformation, from Fundamental Property of Norm on Bounded Linear Transformation, we have:
- $\norm {A x}_\KK \le \norm A \norm x_\HH$
for all $x \in \HH$.
Similarly, since $B$ is a bounded linear transformation we have:
- $\norm {B x}_\KK \le \norm B \norm x_\HH$
for all $x \in \HH$.
Let $x \in \HH$.
Then, we have:
\(\ds \norm {\paren {A + B} x}_\KK\) | \(=\) | \(\ds \norm {A x + B x}_\KK\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds \norm {A x}_\KK + \norm {B x}_\KK\) | Definition of Norm on Vector Space | |||||||||||
\(\ds \) | \(\le\) | \(\ds \norm A \norm x_\HH + \norm B \norm x_\HH\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {\norm A + \norm B} \norm x_\HH\) |
So, taking $c = \norm A +\norm B$, we have:
- $\norm {\paren {A + B} x}_\KK \le c \norm x_\HH$
for all $x \in \HH$.
So:
- $A + B$ is a bounded linear transformation.
Note that:
- $\norm A + \norm B \in \set {c > 0: \forall h \in \HH: \norm {\paren {A + B} h}_\KK \le c \norm h_\HH}$
while, by the definition of the norm, we have:
- $\norm {A + B} = \inf \set {c > 0: \forall h \in \HH: \norm {\paren {A + B} h}_\KK \le c \norm h_\HH}$
So, by the definition of infimum:
- $\norm {A + B} \le \norm A + \norm B$
$\blacksquare$