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Homework Help: Exponential rules

  1. Jul 24, 2006 #1
    Can someone give me a hint on how to prove that

    exp[A+B]=exp[A]expexp[-c/2]

    where A and B are two operators such that [A,B]=c, where c is a complex number.

    This is not homework or something, I'm just curious when reading the rules.
     
  2. jcsd
  3. Jul 25, 2006 #2

    nazzard

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    Gold Member

    Hello Logarythmic,

    the identity would also be true if [A,B] commutes with both A and B. c doesn't necessarily have to be a complex number in the first place.

    A common proof starts with defining:

    [tex]f(x)=e^{Ax}\,e^{Bx}[/tex]

    Calculating [itex]\frac{df}{dx}[/itex] (*) will lead to a first-order differential equation. After finding the solution, f(1) will give the identity.

    (*)hint: if [A,B] commutes with A and B then:

    [tex]e^{-Bx}\,A\,e^{Bx}=A+x[A,B][/tex]

    Regards,

    nazzard
     
  4. Jul 25, 2006 #3
    Thanks for your answer but I don't get it at all. When calculating the derivative of f(x), should I keep in mind that the operators could depend on x? If A and B commutes, then I could prove it just like with ordinary numbers, right? So suppose that A and B do not commute. Calculating df/dx and solving the differential equation just gives me the originally defined f(x)...? Isn't the easiest way to prove it by using the series expansion of e^A?
     
  5. Jul 25, 2006 #4

    nazzard

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    Treat the operators A and B as if they would not depend on x:

    [tex]\frac{d}{dx}(e^{Ax}\,e^{Bx})=e^{Ax}\,A\,e^{Bx}+e^{Ax}\,e^{Bx}\,B[/tex]

    [tex]=e^{Ax}\,e^{Bx}\,e^{-Bx}\,A\,e^{Bx}+e^{Ax}\,e^{Bx}\,B[/tex]

    Now you can factor out f(x) and apply (*).
     
    Last edited: Jul 25, 2006
  6. Jul 29, 2006 #5
    I solved it. Thanks. =)
     
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