HW5 (MTH847 Fall 2017)

General Info

Remember: for every question you must indicate all the people with whom you collaborated. If you didn't collaborate with anyone else on a given problem, write "Collaborators: none."

Please write legibly, or type-up your solutions in LaTeX. The grader will greatly appreciate it.

This problem set will be due in class, Wednesday November 15.

Problems

  1. The conclusions of question 9 of HW4 is true for all odd dimensions 3. The first part, however, fails when d=1:
    Suppose u:R×RR solves 2ttu+2xxu=f with data u(0,x)=g,tu(0,x)=h where fCc(R×R) and gCc(R) and hCc(R). Prove that, if h0 and f0 are such that they are not both identically vanishing, then for every x0R the function tu(t,x0) has noncompact support.
  2. Consider the nonlinear wave equation on R×R: u=λupu(0,x)=g(x)tu(0,x)=h(x) where λR and p2 is an integer. We can rewrite this using Duhamel's formula applied to D'Alembert's solution as the integral equation u(t,x)=12[g(x+t)+g(xt)+x+txth(y) dy]12t0x+(ts)x(ts)λu(s,y)p dy ds. Prove that for every g,hC0c(R), there exists a time T and a function uC0([0,T]×R) such that u verifies the integral equation given above.
    (Hint: emulate our proof of local existence for the nonlinear heat equation. Construct a sequence of functions by setting ui+1(t,x)=12[g(x+t)+g(xt)+x+txth(y) dy]12t0x+(ts)x(ts)λui(s,y)p dy ds and showing that the sequence converges uniformly on [0,T]×R if T is chosen small enough.)
    Solution to this question is now available
  3. (This question uses the same notation as HW4 question 10). Recall from class that if v=(v0,v1,,vd) is a vector field, we define its deformation tensor to be (v)πjk=di=0mikxivj+mijxivk. Suppose v is such that its deformation tensor vanishes identically.
    • Prove that the components of v satisfies vi=0.
    • Prove that whenever ϕ solves the wave equation, so does the quantity ψ=di=0vixiϕ.
  4. Let v,w be two time-like vectors. Prove that
    • vw0
    • the sign of vw is determined by whether the v0 and w0 components have the same or opposite signs.
  5. Let v,w be any two time-like vectors, such that vw>0. Prove that (v)Jw0 with equality if and only if ϕ=0.
  6. Suppose ϕ,ψC2(R×Rd) both solve the wave equation. Suppose further that on the set {t2+|x|21} the two solutions agree. What is the largest space-time subset on which you can guarantee that ϕ=ψ?
  7. Let ϕC2(R×Rd) solve the homogeneous wave equation. Suppose we know that along the set (1,x):x1<λ(1,x):x1>λ both ϕ and tϕ vanish.
    • Prove that if λ1 then ϕ0.
    • Prove that if λ<1 there exists a ϕ, not identically vanishing, satisfying the conditions set out above.
  8. Suppose ϕC4(R×Rd) solves the homogeneous wave equation, with boundary conditions ϕ(0,x)=3tttϕ(0,x)=0. Suppose further that tϕ(0,x) is a bounded function on Rd. Prove that there exists a constant c such that ϕ(t,x)=ct.
    (Hint: first study the function ψ(t,x)=2ttϕ(t,x).)
  9. Let ΩRd be a bounded open set with C1 boundary. Consider the system ϕ=0 on (0,T)×Ω with initial-boundary conditions ϕ(0,x)=tϕ(0,x)=0,xΩ and tϕ+nϕ=0 on [0,T]×Ω, where nϕ denotes the outward normal derivative on Ω. Prove, using the energy method, that if ϕC2([0,T]ׯΩ) solves the above system, then ϕ0 on [0,T]ׯΩ.
    (Remark: the boundary condition tϕ+nϕ=0 is known as the "no incoming radiation" boundary condition.)
  10. Prove that the "time-reversed" version of the previous question is false in general. More precisely, find ϕC2(R×R) such that ϕ=0; ϕ(1,x)=tϕ(1,x)=0 for every x[1,1]; tϕ+nϕ=0 along [0,1]×1,1; but ϕ(0,x) on [-1,1].
    (Remark: as the name indicates, the "no incoming radiation" boundary condition prevents waves flowing into the domain from outside the domain, but it does not prevent waves flowing out of the domain from the inside.)
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