Homework Set # 12 - Not Due – Solutions Available on November 25
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1) Let F(ω) be the Fourier (exponential) transform of f(x) and G(ω) be the Fourier transform of g(x)=f(x+a). Show that G(ω)=e-iaωF(ω).
2) 20.2.2
3) 20.2.4
4)
20.2.7
5)
20.2.8
6)
20.3.6
7)
For a point source at the origin, the three-dimensional neutron
diffusion equation becomes
-D∆2φ(r)+K2Dφ(r)=Qδ(r),
∆2 stands for the Laplacian. Apply a three-dimensional Fourier transform. Solve the transformed equation. Transform the solution back into r-space.
Solution
(pdf format)
8)
a) Consider the function f(x)=1 for |x|<1 and 0
otherwise.
1) Can you expand f(x) in terms of a sine Fourier transform?
Why?
2) What
Fourier transform should you use? Calculate this FT for f(x).
b) Now consider f(x)=1 for a<x<b with a>0 and b>0 and
f(x)=0 otherwise.
1) Can you expand f(x) in terms of a sine FT? Why?
2) If the answer to the previous question is yes perform the
FT.
3) For
what values of x is your sine transform appropriated?
4) What assumption are you making about f(-x)?
5) Can you expand f(x) as a cosine FT? What assumption do you need
to make about f(-x)?
6) If you wanted to obtain a FT of f(x) valid for all x what FT
should you use? Why? Find the coefficients .