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SOLDE with delta function

  1. May 19, 2013 #1
    Hi there, my version of Mathematica may be too old and i'm not finding this one by hand so any help would be appreciated:
    ψ''(z)=[k2/4 –M2 –kδ(z)]ψ(z),
    where δ(z) is the Dirac delta, k and M constants.

    i can solve the same equation without the M^2 term by exp(k|z|/2), but this one proves to be much more complicated.

    Please note that this is part of a problem where solving the D.E. is actually not the issue: i'm supposed to use a software or anything that helps so i'm really just trying to get the answer.. thanks!!
  2. jcsd
  3. May 19, 2013 #2
    Version 9.0.1 DSolve can't crack it.

    Perhaps this will help

    In[1]:= k = 1; m = 2;
    f = s[z] /. NDSolve[{s''[z] == (k^2/4 - m^2 - k DiracDelta[z]) s[z], s[0] == 0, s'[0] == 1/2}, s[z], {z, 0, 10}]

    Out[2]= InterpolatingFunction[{{0., 10.}}, <>][z]}

    In[3]:= Plot[f, {z, 0, 10}]

    Out[3]= ...PlotSnipped...
  4. May 19, 2013 #3
    Thank you Bill, i'm going to look into this "interpolating function"...
    Actually, exp[–k|z|/2]cos(Mz) almost works but i pick up an extra term in the D.E.; maybe an infinite series would do the job, i just can't find it so far...
  5. May 19, 2013 #4
    you dont need Mathematica or anything, just solve the simple second order DE for z<0 and z>0 (the delta function term is zero here) and then match them at z=0. The function itself should be continuous and then there is a jump in the derivative at z=0 due to the delta function.
  6. May 19, 2013 #5
    Thank you, you're right. It's good to remember what these functions really are sometimes!..
    I'm going to try that.
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