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Integral of e^t*H(t)?

  1. May 21, 2013 #1
    1. The problem statement, all variables and given/known data

    -∞ H(t)*e-2tdt

    2. Relevant equations

    See above.

    3. The attempt at a solution

    I know if I were just integrating H(t) by itself, I would get a ramp function. I also know e^-2t by itself will not converge for the given limits of +/- infinity. I just want to know what is going on when the two multiply together, and if integration by parts would serve me here (if it is even necessary?)?

    I have tried to evaluate it with my graphing calculator, and it seems to say the answer is 1/2, though I'm not sure how it has arrived there.

    Any help would be greatly appreciated!
     
  2. jcsd
  3. May 21, 2013 #2

    Dick

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    H(t) is a step function. It's 1 if t>=0 and 0 if t<0. So H(t)f(t)=f(t) if t>=0 and 0 if t<0.
     
  4. May 21, 2013 #3
    Why does my calculator give 1/2, then? If I follow your logic, then I should plug in my limits of inf and -inf to e^(-2t), and this gives me 0 -0.
     
  5. May 21, 2013 #4

    Dick

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    e^(-2t) for the limits inf and -inf diverges, like you said before. I mean the function you are integrating vanishes where t is in (-inf,0) so you can ignore that part. Just use 0 as the lower bound instead of -inf.
     
  6. May 21, 2013 #5
    Ahh, I totally understand now! Thanks for that last bit, it helped give my mind a kick. Essentially it's the same as integrating from 0 to inf of just e^-2t given that H(t) is 1 starting from 0 and going on to infinity! Thanks!
     
  7. May 21, 2013 #6

    Ray Vickson

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    You are just evaluating the elementary integral
    [tex] \int_0^{\infty} e^{-2t} \, dt. [/tex]
    That's what H does: it zeros-out the part from t = -∞ to t = 0.
     
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