# Linear Functionals - Continuity and Boundedness

## Homework Statement

Prove that a continuous linear functional, $$f$$ is bounded and vice versa.

## Homework Equations

I know that the definition of a linear functional is:
$$f( \alpha|x> + \beta|y>) = \alpha f(|x> ) + \beta f( |y> )$$
and that a bounded linear functional satisfies:
$$||f(|x>)) || \leq \epsilon ||\ |x> || , \ \ \ \epsilon > 0$$

## The Attempt at a Solution

I tried the following by letting:

$$f(|x>) = \sum a_{i}f( |x_{i}> )$$
then applying triangle inequality:
$$|| f(|x>) || \leq \sum||a_{i}|| \ || f( |x_{i}> ) ||$$

but now I'm stuck, can someone please help get going in right direction? thanks!

## Answers and Replies

dextercioby
Science Advisor
Homework Helper
This is a standard functional analysis thing which is valid in normed/preBanach spaces and is normally presented in books as a result applicable to functionals when seen as operators from a normed space to C/R seen as normed spaces wrt the modulus.

So the proof for operators goes in 2 steps. First you show that a linear operator continuous at a point is continuous everywhere on the normed space it acts. Then you can show the desired equivalence.

This is neatly proved in Prugovecky's <Quantum Mechanics in Hilbert Space>, Ch.III