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Homework Help: Linear Functionals - Continuity and Boundedness

  1. Nov 2, 2011 #1
    1. The problem statement, all variables and given/known data

    Prove that a continuous linear functional, [tex] f [/tex] is bounded and vice versa.

    2. Relevant equations

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

    3. The attempt at a solution

    I tried the following by letting:

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

    but now I'm stuck, can someone please help get going in right direction? thanks!
  2. jcsd
  3. Nov 3, 2011 #2


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    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
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