课程名称︰物理数学 课程性质︰选修 课程教师︰江衍伟 开课学院：电机资讯学院 开课系所︰电信工程学研究所 考试日期（年月日）︰106.1.11 考试时限（分钟）：170 是否发放奖励金：是 试题 : 1. Consider a differential equation u"(t) - k^2*u(t) = f(t), 0<t<∞, with the initial conditions u(0)=u'(0)=0, where k is a real constant and f(t) is a given function. (a) Find the Green's function g(t,τ) of this problem, satisfying g"(t,τ) - k^2g(t,τ) = δ(t,τ). [6%] (b) Express the solution u(t) in terms of g(t,τ) and f(t), and show that the causality is satisfied. [4%] 2. (a) Consider the eigenproblem u_n"(x) + k*u_n(x) = λ_n*u_n(x), 0<x<1, u_n(0)=u_n(1)=0, where k is a nonzero real constant. Find the eigenfunctions u_n(x) and eigenvalues λ_n. [8%] (b) By using the eigenfunction expansion method and the result of (a), solve the following boundary value problem: u"(x) + k*u(x) = sin(πx), 0<x<1, u(0)=3, u(1)=5, where k is a nonzero real constant. [8%] (c) Does your solution in (b) exactly satisfy the boundary conditions at x=0 and x=1? If no, discuss the reason briefly. [2%] (d) Is your solution valid for all nonzero real k? If no, discuss the reason briefly. [2%] 3. Find the function y(x) which makes the following functional stationary: I = integral[0→1](2y'^2-y'y-5y'+y)dx + [y(0)]^2 - 2y(0) - 2[y(1)]^2 + 4y(1) (a) when y(0)=2 and y(1)=3. [10%] (b) when the values y(0) and y(1) are not specified. [10%] 4. (a) For a linear differential operator L, we can consider the Green's function G(x,ξ;λ) satisfying LG-λG=δ(x-ξ). Discuss briefly the importance of G in investigating the spectral property of operator L. [5%] (b) Why do we seldom consider the residual spectrum for a practical boundary value problem? Explain briefly. [5%] (c) Please give a physical or engineering example for which a continuous spectrum exists, and discuss its physical meaning briefly. [5%] 5. (a) Consider the integral F(z) = integral[0→∞] e^(-t^3)/(t+z^3) dt for complex valued z. By using Watson's lemma, find the asymptotic expansion of F(z) in terms of {z^-j},j=0→∞ for |z|→∞. [10%] (b) If there is no saddle point along the integration path which has been deformed as much as possible, can we use the method of steepest descent to asymptotically evaluate the integral? Discuss this briefly. [8%] 6. Consider a three-dimensional Poisson equation ▽^2 u(r) = f(r) within a finite region V bounded by a closed surface S, on which a Dirichlet boundary condition is specified. Show that the solution of this problem is unique. [7%] 7. Solve the following partial differential equation for u(x,t) by the characteristic method. [10%] ∂u/∂t + ∂/∂x [6u^2-u] = 0, -∞<x<∞, t>0 u(x,0) = 2x + 5, -∞<x<∞ --

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