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MATH3121 Assignment
1. The Helmholtz equation takes the form, uxx + uyy + k 2u = f(x, y), for −∞ < x <∞, −∞ < y <∞. i) Assuming that the functions u(x, y) and f(x, y) have Fourier transforms show that the solution to this equation can formally be written: u(x, y) = − 1 4π2 ∫ ∞ −∞ ∫ ∞ −∞ ∫ ∞ −∞ ∫ ∞ −∞ e−i(λ(x−ζ)+µ(y−η)) f(ζ, η) λ2 + µ2 − k2 dλ dµ dζ dη. ii) Find a solution to the Helmholtz equation for the special case where f(x, y) = 0. Is this solution consistent with the general result given in part i)? Justify your answer. 2. Using Laplace transforms, or otherwise, find a function f that satisfies the following equation:∫ t 0 exp (−(t− z)) f ′(z)dz = 1. for t > 0, subject to the initial condition f(0) = 2. Substitute your answer back into the equation to verify that it works. Explain any issues that you encounter. 3. Find the inverse Laplace transform of, F (s) = 1 s+ exp(−sτ) , where τ is a positive real parameter. 4. Consider the following PDE, ∂u ∂t − κ∂ 2u ∂x2 = S0δ(x)δ(t), subject to the initial condition, u(x, 0) = δ(x), with κ > 0, and S0 > 0. Find the solution of this PDE by taking both a Fourier and a Laplace transformation. You may use that fact that the Laplace transformation of the Dirac delta function is one, i.e. L{δ(t)} = 1.