MATH 619, Fall 2022, First Midterm
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MATH 619, Fall 2022, First Midterm
— Questions —
1. By definition, we say a function u : R2 → R is a weak solution to the Wave Equation if u is continuous, and if
(⋆) u(x + h,t) + u(x − h,t) = u(x,t + h) + u(x,t − h)
holds for all x,t ∈ R and all h > 0.
We say u is a classical solution if u is C2 and if utt (x,t) = uxx (x,t) holds for all x,t ∈ R.
(a) Give an example of a weak solution that is not a classical solution, and explain your example.
(b) Prove, or give a counterexample to the following statement: if un : R2 → R is a sequence of classical solutions to the Wave Equation, and if un converges uniformly to u : R2 → R then u is also a classical solution to the Wave Equation.
2. Suppose f : R → C is a Riemann integrable 2π-periodic function.
(a) Show that if f is C1 then limk →±∞ kfˆk = 0.
(b) Prove, or give a counterexample: if the Fourier coefficients of f satisfy |fˆk | ⩽ for all k 0, then f is C2 .
3. Let n , n ∈ C be two sequences of complex numbers for which
∞
工 |k | + |k | < ∞ .
k= −∞
Show that the series
∞
u(x,t) = 工 k eikx eikt + k eikx e−ikt
k= −∞
converges uniformly on R2 .
(b) Show that u(x,t) is a weak solution to the Wave Equation.
2022-10-25