Solution to Partial Differential Equations: Methods and Applications (Robert McOwen) Section 3.1
- Solve the initial value problems:
- utt−c2uxx=0 with u(x,0)=x3 and ut(x,0)=sinx.
- utt−c2uxx=2t with u(x,0)=x2 and ut(x,0)=1.
- Using d'Alembert's formula (6) with g(x)=x3 and h(x)=sinx, we have
u(x,t)=(x+ct)3+(x−ct)32+12c∫x+ctx−ctsinξdξ=x3+3c2xt2−cos(x+ct)−cos(x−ct)2c=x3+3c2xt2+sin(x)sin(ct)cfor (x,t)∈R2.
- Since the wave equation is a linear equation, the solution u can be decomposed as u=v+w, where
{vtt−c2vxx=0,v(x,0)=x2, vt(x,0)=1and{wtt−c2wxx=2t,w(x,0)=0, wt(x,0)=0.
By d'Alembert's formula (6) with g(x)=x2 and h(x)=1, we havev(x,t)=(x+ct)2+(x−ct)22+12c∫x+ctx−ct1dξ=x2+c2t2+tfor (x,t)∈R2.
On the other hand, we use (19), which follows from Duhamel's principle and d'Alembert's formula, to getw(x,t)=12c∫t0∫x+c(t−s)x−c(t−s)2sdξds=1c∫t0s⋅2c(t−s)ds=2∫t0(ts−s2)ds=ts2−2s33|t0=t33for (x,t)∈R2.
Therefore, the solution isu(x,t)=v(x,t)+w(x,t)=x2+c2t2+t+t33for (x,t)∈R2.
- Solve the initial/boundary value problem
{utt−uxx=0for 0<x<π and t>0,u(x,0)=0, ut(x,0)=1for 0<x<π,u(0,t)=0, u(π,t)=0for t≥0
using a Fourier series. Using the parallelogram rule, find the values of the solution in the various regions. Is the resulting solution continuous? Is it C1? - If λ>0, we may write λ=μ2 for some μ>0. Then X(x)=c1eμx+c2e−μx. Then by X(0)=X(π)=0, we find c1=c2=0 and then X≡0, which is a trivial solution.
- If λ=0, we get X(x)=c1+c2x. By X(0)=X(π)=0, it is easy to see c1=c2=0 and hence X≡0, which is a trivial solution.
- If λ<0, we may write λ=−μ2 for some μ>0. Then we have X(x)=c1sin(μx)+c2cos(μx). By X(0)=0, we get c2=0. From X(π)=0, we have c1sin(μπ)=0. Since we want to seek the nontrivial solution, we hope c1≠0, which means sin(μπ) should be zero and μ must be an integer.
- R0={(x,t)∈R2:t≤x≤π−t,0≤t≤π/2}.
- R3k−2={(x,t)∈R2:0≤x≤π/2,(k−1)π+x≤t≤kπ−x} for k∈N.
- R3k−1={(x,t)∈R2:π/2≤x≤π,kπ−x≤t≤(k−1)π+x} for k∈N.
- R3k={(x,t)∈R2:(k−1)π≤t−x≤kπ,kπ≤t+x≤(k+1)π} for k∈N.
- u(x,t)=(−1)k−1x in R3k−1 for k∈N.
- u(x,t)=(−1)k−1(π−x) in R3k−2 for k∈N.
- u(x,t)=(−1)k(t−kπ) in R3k for k∈N∪{0}.
- Consider the initial/boundary value problem
{utt−uxx=0for 0<x<π and t>0,u(x,0)=x, ut(x,0)=0for 0<x<π,ux(0,t)=0, ux(π,t)=0for t≥0.
- Find a Fourier series solution, and sum the series in regions bounded by characteristics. Do you think the solution is unique?
- Use the parallelogram rule to solve this problem; is the resulting solution unique? continuous? C1?
- Using the separation of variables, we suppose that the solution u(x,t) is of the form
u(x,t)=∞∑n=0un(x,t)=∞∑n=0Xn(x)Tn(t)for x∈[0,π] and t≥0.
Due to the superposition principle, we firstly consider u(x,t)=X(x)T(t) to fulfill the wave equation with the boundary condition, which givesX(x)T″(t)−X″(x)T(t)=0andX′(0)T(t)=X′(π)T(t)=0.
Surely, we want to find a general solution so T is not identically zero and X′(0)=X′(π)=0. From the differential equation, we have X″(x)/X(x)=T″(t)/T(t) for x∈(0,π) and t>0, which means there exists a constant λ such that X″(x)=λX(x) and T″(t)=λT(t). To find nontrivial solution X, we divide three cases as follows.- If λ>0, we may write λ=μ2 for some μ>0. Then X(x)=c1eμx+c2e−μx. Then by X′(0)=X′(π)=0, we find c1=c2=0 and then X≡0, which is a trivial solution.
- If λ=0, we get X(x)=c1+c2x. By X′(0)=X′(π)=0, it is easy to see c2=0 and hence X≡c1. This gives a nonzero solution, provided c1≠0.
- If λ<0, we may write λ=−μ2 for some μ>0. Then we have X(x)=c1sin(μx)+c2cos(μx). By X′(0)=0, we get c1=0. From X′(π)=0, we have −c2μsin(μπ)=0. Since we want to seek the nontrivial solution, we hope c2≠0, which means sin(μπ) should be zero and μ must be an integer.
u(x,t)=(A+Bt)+∞∑n=1(cnsin(nt)+dncos(nt))cos(nx)for x∈[0,π] and t≥0.
Since ut(x,0)=0 on [0,π], we have0=ut(x,0)=B+∞∑n=1cncos(nx),
which gives B=cn=0 for n∈N. Hence the solution becomesu(x,t)=A+∞∑n=1dncos(nt)cos(nx)for x∈[0,π] and t≥0.
Since u(x,0)=x, we havex=u(x,0)=A+∞∑n=1dncos(nx)for x∈[0,π].
Integrating it over [0,π], we get A=π2. Multiplying it by cos(mx) and then integrating it over [0,π], we obtaindn=∫π0xcos(nx)dx∫π0cos2(nx)dx=xsin(nx)n|π0−1n∫π0sin(nx)dx12∫π0(1−cos(2nx))dx=2((−1)n−1)n2πfor n∈N.
Thus, the Fourier solution must beu(x,t)=π2+2π∞∑n=1(−1)n−1n2cos(nt)cos(nx)=π2−4π∞∑m=1cos[(2m−1)t]cos[(2m−1)x](2m−1)2for x∈[0,π] and t≥0.
Using the product to sum formula, the Fourier solution beomcesu(x,t)=π2−2π∞∑m=1cos[(2m−1)(x+t)]+cos[(2m−1)(x−t)](2m−1)2for x∈[0,π] and t≥0.
To sum the Fourier series in regions bounded by characteristics, we define- R0={(x,t)∈R2:t≤x≤π−t,0≤t≤π/2}.
- R3k−2={(x,t)∈R2:0≤x≤π/2,(k−1)π+x≤t≤kπ−x} for k∈N.
- R3k−1={(x,t)∈R2:π/2≤x≤π,kπ−x≤t≤(k−1)π+x} for k∈N.
- R3k={(x,t)∈R2:(k−1)π≤t−x≤kπ,kπ≤t+x≤(k+1)π} for k∈N.
In region R0 (0<x+t<π and 0<x−t<π), we haveu(x,t)=π2−2π[π2−2π(x+t)8+π2−2π(x−t)8]=x.
In region R3k−2 (0<x+t−(k−1)π<π and 0<x−t+kπ<π), we haveu(x,t)=π2+2(−1)kπ∞∑m=1cos[(2m−1)(x+t−(k−1)π)]−cos[(2m−1)(x−t+kπ)](2m−1)2=π2+2(−1)kπ[π2−2π(x+t−(k−1)π)8−π2−2π(x−t+kπ)8]=π2+(−1)k(−t+kπ−π2).
In region R3k−1 (0<x+t−kπ<π and 0<x−t+(k−1)π<π), we haveu(x,t)=π2−2(−1)kπ∞∑m=1[cos[(2m−1)(x+t−kπ)]−cos[(2m−1)(x−t+(k−1)π)](2m−1)2]=π2−2(−1)kπ[π2−2π(x+t−kπ)8−π2−2π(x−t+(k−1)π)8]=π2+(−1)k(t−kπ+π2).
In region R3k (0<x+t−kπ<π and 0<x−t+kπ<π), we haveu(x,t)=π2−2(−1)kπ∞∑m=1cos[(2m−1)(x+t−kπ)]+cos[(2m−1)(x−t+kπ)](2m−1)2=π2−2(−1)kπ[π2−2π(x+t−kπ)8+π2−2π(x−t+kπ)8]=π2−(−1)k(π−2x4)=2π−(−1)k(π−2x)4.
Clearly, this solution is continuous but not differentiable along the characteristics, and such solution is unique. - To use the parallelogram rule, we can again employ the region Rk for k∈N∪{0} as in (a). Due to the lack of the boundary data ux, it is impossible to determine the solution uniquely. But it is possible to obtain the unique continuous solution (which is equal to the Fourier series solution) and we cannot obtain C1 solution. Surely, we may observe that ux(x,0)=1 for 0<x<π, which is discontinuous at x=0 because ux(0,t)=0 for t≥0. Therefore, the parallelogram rule only gives the solution in region R0:
u(x,t)=u(x+t,0)+u(x−t,0)2+12∫x+tx−tut(s,0)ds=(x+t)+(x−t)2+12∫x+tx−t0ds=x.
Warning: It does not seem reasonable to extend ux continuously so we cannot use the parallelogram rule for ux due to the lack of information of ux in region Rk for k∈N. - Consider the initial boundary value problem
{utt−c2uxx=0for x,t>0,u(x,0)=g(x), ut(x,0)=h(x)for x>0,u(0,t)=0for t≥0,
where g(0)=0=h(0). If we extend g and h as odd functions on −∞<x<∞, show that d'Alembert's formula (6) gives the solution. - Find in closed form (similar to d'Alembert's formula) the solution u(x,t) of
{utt−c2uxx=0for x,t>0,u(x,0)=g(x), ut(x,0)=h(x)for x>0u(0,t)=α(t)for t≥0,
where g,h,α∈C2 satisfy α(0)=g(0), α′(0)=h(0), and α″(0)=c2g″(0). Verify that u∈C2, even on the characteristic x=ct. - Solve the initial/boundary value problem
\left\{\begin{aligned} &u_{tt}-u_{xx}=1\quad\text{for}~0<x<\pi~\text{and}~t>0,\\&u(x,0)=0,~~u_t(x,0)=0\quad\text{for}~0<x<\pi,\\&u(0,t)=0,~~u(\pi,t)=-\pi^2/2\quad\text{for}~t\geq0.\end{aligned}\right.
Describe the singularities (i.e., is u C^2? If not, where does it fail? Is u C^1? etc.). - If \lambda>0, we may write \lambda=\mu^2 for some \mu>0. Then X(x)=c_1e^{\mu x}+c_2e^{-\mu x}. Then by X(0)=X(\pi)=0, we find c_1=c_2=0 and then X\equiv0, which is a trivial solution.
- If \lambda=0, we get X(x)=c_1+c_2x. By X(0)=X(\pi)=0, it is easy to see c_1=c_2=0 and hence X\equiv0, which is a trivial solution.
- If \lambda<0, we may write \lambda=-\mu^2 for some \mu>0. Then we have X(x)=c_1\sin(\mu x)+c_2\cos(\mu x). By X(0)=0, we get c_2=0. From X(\pi)=0, we have c_1\sin(\mu\pi)=0. Since we want to seek the nontrivial solution, we hope c_1\neq0, which means \sin(\mu\pi) should be zero and \mu must be an integer.
- R_0=\{(x,t)\in\mathbb R^2\,:\,t\leq x\leq\pi-t,\,0\leq t\leq\pi/2\}.
- R_{3k-2}=\{(x,t)\in\mathbb R^2\,:\,0\leq x\leq\pi/2,\,(k-1)\pi+x\leq t\leq k\pi-x\} for k\in\mathbb N.
- R_{3k-1}=\{(x,t)\in\mathbb R^2\,:\,\pi/2\leq x\leq\pi,\,k\pi-x\leq t\leq(k-1)\pi+x\} for k\in\mathbb N.
- R_{3k}=\{(x,t)\in\mathbb R^2\,:\,(k-1)\pi\leq t-x\leq k\pi,\,k\pi\leq t+x\leq(k+1)\pi\} for k\in\mathbb N.
- Use Fourier series to solve the initial/boundary value problem for the Klein-Gordon equation
\left\{\begin{aligned} &u_{tt}-c^2u_{xx}+m^2u=0\quad\text{for}~0<x<\pi~\text{and}~t>0,\\&u(x,0)=g(x),~~u_t(x,0)=h(x)\quad\text{for}~0<x<\pi,\\&u(0,t)=0=u(\pi,t)\quad\text{for}~t>0.\end{aligned}\right.
Notice that the solution u(x,t) is bounded as t\to\infty. - Do the same for the equation u_{tt}-c^2u_{xx}-m^2u=0. If c^2<m^2, show that the solution could be unbounded as t\to\infty.
- Use Fourier series to solve the initial/boundary value problem for the Klein-Gordon equation
- Using the separation of variables, we suppose that the solution u(x,t) is of the form
\displaystyle u(x,t)=\sum_{n=1}^\infty u_n(x,t)=\sum_{n=1}^\infty X_n(x)T_n(x)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0.
Due to the superposition principle, we firstly consider u(x,t)=X(x)T(t) to fulfill the Klein-Gordon equation with the boundary condition, which givesX(x)T''(t)-c^2X''(x)T(t)+m^2X(x)T(t)=0\quad\text{and}\quad X(0)T(t)=X(\pi)T(t)=0.
Surely, we want to find a general solution so T is not identically zero and X(0)=X(\pi)=0. From the differential equation, we have\displaystyle\frac{X''(x)}{X(x)}=\frac1{c^2}\frac{T''(t)}{T(t)}+\frac{m^2}{c^2}\quad\text{for}~x\in(0,\pi)~\text{and}~t\in(0,\infty),
which means there exists a constant \lambda such that X''(x)=\lambda X(x) and T''(t)+(m^2-\lambda c^2)T(t)=0. To find nontrivial solution X, we divide three cases as follows.- If \lambda>0, we may write \lambda=\mu^2 for some \mu>0. Then X(x)=c_1e^{\mu x}+c_2e^{-\mu x}. Then by X(0)=X(\pi)=0, we find c_1=c_2=0 and then X\equiv0, which is a trivial solution.
- If \lambda=0, we get X(x)=c_1+c_2x. By X(0)=X(\pi)=0, it is easy to see c_1=c_2=0 and hence X\equiv0, which is a trivial solution.
- If \lambda<0, we may write \lambda=-\mu^2 for some \mu>0. Then we have X(x)=c_1\sin(\mu x)+c_2\cos(\mu x). By X(0)=0, we get c_2=0. From X(\pi)=0, we have c_1\sin(\mu\pi)=0. Since we want to seek the nontrivial solution, we hope c_1\neq0, which means \sin(\mu\pi) should be zero and \mu must be an integer.
T(t)=c_n\cos(t\sqrt{m^2+c^2n^2})+d_n\sin(t\sqrt{m^2+c^2n^2})\quad\text{for}~n\in\mathbb N.
Thus, the solution u has the following form:\displaystyle u(x,t)=\sum_{n=1}^\infty[c_n\cos(t\sqrt{m^2+c^2n^2})+d_n\sin(t\sqrt{m^2+c^2n^2})]\sin(nx)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0.
From u(x,0)=g(x) and u_t(x,0)=h(x) for x\in(0,\pi), we have\left\{\begin{aligned} &g(x)=u(x,0)=\sum_{n=1}^\infty c_n\sin(nx),\\&h(x)=\sum_{n=1}^\infty d_n\sqrt{m^2+c^2n^2}\sin(nx).\end{aligned}\right.
Multiplying them by \sin(kx) and then integrating them over [0,\pi], we can obtain\displaystyle c_n=\frac2\pi\int_0^\pi\!g(x)\sin(nx)\,\mathrm dx,\quad d_n=\frac2{\pi\sqrt{m^2+c^2n^2}}\int_0^\pi\!h(x)\sin(nx)\,\mathrm dx\quad\text{for}~n\in\mathbb N.
Therefore, the solution is\displaystyle u(x,t)=\frac2\pi\sum_{n=1}^\infty\left[\cos(t\sqrt{m^2+c^2n^2})\int_0^\pi\!g(x)\sin(nx)\,\mathrm dx+\frac{\sin(t\sqrt{m^2+c^2n^2})}{\sqrt{m^2+c^2n^2}}\int_0^\pi\!h(x)\sin(nx)\,\mathrm dx\right]\sin(nx)
for x\in(0,\pi) and t>0. Due to he Parseval's Theorem, we can estimate the sum of squares of c_n and d_n, i.e., \displaystyle\sum_{n=1}^\infty c_n^2 and \displaystyle\sum_{n=1}^\infty d_n^2 is bounded, provided that g and h are L^2-functions. This implies u is bounded as t\to\infty. - Using the separation of variables, we suppose that the solution u(x,t) is of the form
\displaystyle u(x,t)=\sum_{n=1}^\infty u_n(x,t)=\sum_{n=1}^\infty X_n(x)T_n(x)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0.
Due to the superposition principle, we firstly consider u(x,t)=X(x)T(t) to fulfill the Klein-Gordon equation with the boundary condition, which givesX(x)T''(t)-c^2X''(x)T(t)-m^2X(x)T(t)=0\quad\text{and}\quad X(0)T(t)=X(\pi)T(t)=0.
Surely, we want to find a general solution so T is not identically zero and X(0)=X(\pi)=0. From the differential equation, we have\displaystyle\frac{X''(x)}{X(x)}=\frac1{c^2}\frac{T''(t)}{T(t)}-\frac{m^2}{c^2}\quad\text{for}~x\in(0,\pi)~\text{and}~t\in(0,\infty),
which means there exists a constant \lambda such that X''(x)=\lambda X(x) and T''(t)-(m^2+\lambda c^2)T(t)=0. To find nontrivial solution X, we divide three cases as follows.- If \lambda>0, we may write \lambda=\mu^2 for some \mu>0. Then X(x)=c_1e^{\mu x}+c_2e^{-\mu x}. Then by X(0)=X(\pi)=0, we find c_1=c_2=0 and then X\equiv0, which is a trivial solution.
- If \lambda=0, we get X(x)=c_1+c_2x. By X(0)=X(\pi)=0, it is easy to see c_1=c_2=0 and hence X\equiv0, which is a trivial solution.
- If \lambda<0, we may write \lambda=-\mu^2 for some \mu>0. Then we have X(x)=c_1\sin(\mu x)+c_2\cos(\mu x). By X(0)=0, we get c_2=0. From X(\pi)=0, we have c_1\sin(\mu\pi)=0. Since we want to seek the nontrivial solution, we hope c_1\neq0, which means \sin(\mu\pi) should be zero and \mu must be an integer.
T_n(t)=\begin{cases}c_n\cos(t\sqrt{m^2+c^2n^2})+d_n\sin(t\sqrt{m^2+c^2n^2})&\text{if}~n>m/c;\\c_n+d_nt&\text{if}~n=m/c;\\c_n\exp(t\sqrt{m^2-c^2n^2})+d_n\exp(-t\sqrt{m^2-c^2n^2})&\text{if}~1\leq n<m/c.\end{cases}
Thus, the solution u has the following form:\displaystyle u(x,t)=\sum_{n=1}^\infty T_n\sin(nx)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0.
We can determine all coefficients of c_n and d_n as for part (a). When c<m, we may observe that T_1 includes the exponential function and becomes unbounded as t tends to infinity. - Derive a formula similar to (6) for the pure initial value problem for the Klein-Gordon equation u_{tt}-c^2u_{xx}+m^2u=0.
Solution
Solution
Using the separation of variables, we suppose that the solution u(x,t) is of the formu(x,t)=∞∑n=1un(x,t)=∞∑n=1Xn(x)Tn(t)for x∈[0,π] and t≥0.
Due to the superposition principle, we firstly consider u(x,t)=X(x)T(t) to fulfill the wave equation with the boundary condition, which givesX(x)T″(t)−X″(x)T(t)=0andX(0)T(t)=X(π)T(t)=0.
Surely, we want to find a general solution so T is not identically zero and X(0)=X(π)=0. From the differential equation, we have X″(x)/X(x)=T″(t)/T(t) for x∈(0,π) and t>0, which means there exists a constant λ such that X″(x)=λX(x) and T″(t)=λT(t). To find nontrivial solution X, we divide three cases as follows.u(x,t)=∞∑n=1(cnsin(nt)+dncos(nt))sin(nx)for x∈[0,π] and t≥0.
Since u(x,0)≡0 on [0,π], we have0=u(x,0)=∞∑n=1dnsin(nx)for x∈[0,π],
which gives dn=0 for n∈N. Differentiating u with respect to t yieldsut(x,t)=∞∑n=1ncncos(nt)sin(nx).
From ut(x,0)=1, we have1=ut(x,0)=∞∑n=1ncnsin(nx)for x∈[0,π].
Multiplying it by sin(mx) and then integrating it over [0,π], we obtain1−(−1)mm=∫π01⋅sin(mx)dx=∫π0∞∑n=1ncnsin(nx)sin(mx)dx=m∫π0cmsin2(mx)dx=mcm2∫π0(1−cos(2mx))dx=mcmπ2.
This means cn=2(1−(−1)n)πn2 for n∈N. Therefore, we obtainu(x,t)=2π∞∑n=11−(−1)nn2sin(nt)sin(nx)=4π∞∑m=1sin[(2m−1)t]sin[(2m−1)x](2m−1)2for x∈[0,π],t≥0.
To use the parallelogram rule, we can firstly define the following regions.
u(x,t)=u(x+t,0)+u(x−t,0)2+12∫x+tx−tut(s,0)ds=t.
In region R1, by the parallelogram rule, the solution isu(x,t)=u(0,−x+t)+u(x+t2,x+t2)−u(−x+t2,−x+t2)=0+x+t2−−x+t2=x.
In region R2, by the parallelogram rule, the solution isu(x,t)=u(x−t+π2,−x+t+π2)+u(π,x+t−π)−u(−x−t+3π2,x+t−π2)=−x+t+π2+0−x+t−π2=π−x.
In region R3, by the parallelogram rule, the solution isu(x,t)=u(x−t+π2,−x+t+π2)+u(x+t2,x+t2)−u(π2,π2)=x−t+π2+(π−x+t2)−π2=π−t.
In region R4, by the parallelogram rule, the solution isu(x,t)=u(0,−x+t)+u(x+t−π2,x+t+π2)−u(−x+t−π2,−x+t+π2)=0+(π−x+t+π2)−(π−−x+t+π2)=−x.
In region R5, by the parallelogram rule, the solution isu(x,t)=u(x−t+2π2,−x+t+2π2)+u(π,x+t−π)−u(−x−t+4π2,x+t2)=(π−−x+t+2π2)+0−(π−x+t2)=x−π.
In region R6, by the parallelogram rule, the solution isu(x,t)=u(x−t+2π2,−x+t+2π2)+u(x+t−π2,x+t+π2)−u(π2,3π2)=−x−t+2π2+(x+t−π2−π)−(π−3π2)=t−2π.
Therefore, we may use the mathematical induction to show that u(x,t+4π)=u(x,t) for t≥0, i.e., we haveRemark: We are able to sum the Fourier series solution in regions bounded by characteristics as follows. By the product to sum formula, the solution can be expressed as
u(x,t)=2π∞∑m=1cos[(2m−1)(x−t)]−cos[(2m−1)(x+t)](2m−1)2for x∈[0,π],t≥0.
Note that ∞∑m=1cos[(2m−1)x](2m−1)2=π2−2πx8 for x∈[0,π] (see the details from Solution of Exercise 3).In region R0 (0<x−t<π and 0<x+t<π), we have
u(x,t)=2π[π2−2π(x−t)8−π2−2π(x+t)8]=t.
In region R3k−2 (0<x−t+kπ<π and 0<x+t−(k−1)π<π), we haveu(x,t)=2(−1)kπ∞∑m=1cos[(2m−1)(x−t+kπ)]+cos[(2m−1)(x+t+(k−1)π)](2m−1)2=2(−1)kπ[π2−2π(x−t+kπ)8+π2−2π(x+t−(k−1)π)8]=(−1)k−1x.
In region R3k−1 (0<x−t+(k−1)π<π and 0<x+t−kπ<π), we haveu(x,t)=2(−1)k−1π∞∑m=1cos[(2m−1)(x−t+(k−1)π)]+cos[(2m−1)(x+t−kπ)](2m−1)2=2(−1)k−1π[π2−2π(x−t+(k−1)π)8+π2−2π(x+t−kπ)8]=(−1)k−1(π−x).
In region R3k (0<x−t+kπ<π and 0<x+t−kπ<π), we haveu(x,t)=2(−1)kπ∞∑m=1cos[(2m−1)(x−t+kπ)]−cos[(2m−1)(x+t−kπ)](2m−1)2=2(−1)kπ[π2−2π(x−t+kπ)8−π2−2π(x+t−kπ)8]=(−1)k(t−kπ).
Solution
Solution
Let ˜g:R→R and ˜h:R→R be defined by˜g(x)={g(x)if x≥0;−g(−x)if x<0,and˜h(x)={h(x)if x≥0;−h(−x)if x<0.
Then we firstly consider the following initial value problem{˜utt−c2˜uxx=0for x∈R and t>0,˜u(x,0)=˜g(x),˜ut(x,0)=˜h(x)for x∈R.
By d'Alembert's formula, the solution ˜u is˜u(x,t)=˜g(x+ct)+˜g(x−ct)2+12c∫x+ctx−ct˜h(s)dsfor (x,t)∈R×ˉR+.
When x=0, it is easy to verify that˜u(0,t)=˜g(ct)+˜g(−ct)2+12c∫ct−ct˜h(s)ds=g(ct)−g(ct)2+12c∫0−ct[−h(−s)]ds+12c∫ct0h(s)ds=0
Thus, function u=˜u|ˉR2+ solves the initial boundary value problem. Moreover, the solution u can be represented asu(x,t)={g(x+ct)+g(x−ct)2+12c∫x+ctx−cth(s)dsif x≥ct;g(ct+x)−g(ct−x)2+12c∫ct+xct−xh(s)dsif 0≤x<ct.
Solution 1
Let v:ˉR2+→R be defined by v(x,t)=u(x,t)−α(t). Then v satisfies{vtt−c2vxx=−α″(t)for (x,t)∈R2+,v(x,0)=g(x)−α(0),vt(x,0)=h(x)−α′(0)for x∈R+,v(0,t)=0for t∈ˉR+.
Set ˜g(x)=g(x)−α(0) and ˜h(x)=h(x)−α′(0). By the assumption, we have ˜g(0)=0 and ˜h(0)=0. Now we consider the following two initival boundary value problems{ϕtt−c2ϕxx=0for (x,t)∈R2+,ϕ(x,0)=˜g(x),ϕt(x,0)=˜h(x)for x∈R+,ϕ(0,t)=0for t∈ˉR+and{φtt−c2φxx=−α″(t)for (x,t)∈R2+,φ(x,0)=φt(x,0)=0for x∈R+,φ(0,t)=0for t∈ˉR+.
Then the solution u(x,t)=v(x,t)+α(t)=ϕ(x,t)+φ(x,t)+α(t) for (x,t)∈ˉR2+. By the result of problem 4, we getϕ(x,t)={˜g(x+ct)+˜g(x−ct)2+12c∫x+ctx−ct˜h(s)dsif x≥ct;˜g(ct+x)−˜g(ct−x)2+12c∫ct+xct−x˜h(s)dsif 0≤x<ct.={−α(0)−tα′(0)+g(x+ct)+g(x−ct)2+12c∫x+ctx−cth(s)dsif x≥ct;−xcα′(0)+g(ct+x)−g(ct−x)2+12c∫ct+xct−xh(s)dsif 0≤x<ct.
On the other hand, we extend φ to ˜φ:R×ˉR+→R by˜φ(x,t)={φ(x,t)if x≥0,t≥0;−φ(−x,t)if x<0,t≥0.
Then ˜φ satisfies{˜φtt−c2˜φxx=−sgn(x)α″(t)for (x,t)∈R×R+,˜φ(x,0)=˜φt(x,0)=0for x∈R.
Now we can use (19), which follows from Duhamel's principle and d'Alembert's formula, to get˜φ(x,t)=12c∫t0∫x+c(t−s)x−c(t−s)[−sgn(ξ)α″(s)]dξds
Since ˜φ(0,t)=0, we arrive at φ=˜φ|ˉR2+. When x≥ct≥0, we have x−ct+cs≥0 for 0≤s≤t and henceφ(x,t)=−12c∫t0∫x+c(t−s)x−c(t−s)α″(s)dξds=−∫t0(t−s)α″(s)ds=−t∫t0α″(s)ds+∫t0sα″(s)ds=−tα′(s)|t0+sα′(s)|t0−∫t0α′(s)ds=−tα′(t)+tα′(0)+tα′(t)−0−α(t)+α(0)=tα′(0)−α(t)+α(0).
When 0≤x<ct, we have x−ct+cs≥0 for (ct−x)/c≤s≤t and x−ct+cs<0 for 0≤s<(ct−x)/c, and henceφ(x,t)=12c∫(ct−x)/c0∫x+c(t−s)x−c(t−s)[−sgn(ξ)α″(s)]dξds+12c∫t(ct−x)/c∫x+c(t−s)x−c(t−s)[−sgn(ξ)α″(s)]dξds=12c∫(ct−x)/c0[∫0x−c(t−s)α″(s)dξ−∫x+c(t−s)0α″(s)dξ]ds−∫t(ct−x)/c(t−s)α″(s)ds=−xc∫(ct−x)/c0α″(s)ds−t∫t(ct−x)/cα″(s)ds+∫t(ct−x)/csα″(s)ds=xc[α′(0)−α′(t−xc)]+t[α′(t−xc)−α′(t)]+[tα′(t)−(t−xc)α′(t−xc)]−[α(t)−α(t−xc)]=xcα′(0)−α(t)+α(t−xc)
Hence the solution u isu(x,t)={g(x+ct)+g(x−ct)2+12c∫x+ctx−cth(s)dsif x≥ct≥0;α(t−xc)+g(ct+x)−g(ct−x)2+12c∫ct+xct−xh(s)dxif 0≤x<ct.
Since g,h,α∈C2, it is clear that u∈C2 in ˉR2+−{(x,t)∈R2+:x=ct,x≥0} and satisfiesux(x,t)={g′(x+ct)+g′(x−ct)2+h(x+ct)−h(x−ct)2cif x>ct≥0;−1cα′(t−xc)+g′(ct+x)+g′(ct−x)2+h(ct+x)+h(ct−x)2cif 0<x<ct,ut(x,t)={c[g′(x+ct)−g′(x−ct)]2+h(x+ct)+h(x−ct)2if x>ct>0;α′(t−xc)+c[g′(ct+x)−g′(ct−x)]2+h(ct+x)−h(ct−x)2if 0<x<ct,uxx(x,t)=1c2utt(x,t)={g″(x+ct)+g″(x−ct)2+h′(x+ct)−h′(x−ct)2cif x>ct>0;1c2α″(t−xc)+g″(ct+x)−g″(ct−x)2+h′(ct+x)−h′(ct−x)2cif 0<x<ct,uxt(x,t)=utx(x,t)={c[g″(x+ct)−g″(x−ct)]+h′(x+ct)+h′(x−ct)2if x>ct>0;−1cα″(t−xc)+c[g″(ct+x)+g″(ct−x)]+h′(ct+x)+h′(ct−x)2if 0<x<ct.
Since α(0)=g(0), it follows that u∈C0(ˉR2+). To show u∈C2, even on the characteristic x=ct, we need to compute the second order derivatives and prove that they are continuous on the line x=ct. For any point (x0,t0) satisfying x0=ct0 for t0>0, we may use the definition of partial derivative with respect to x to compute the right derivative and left derivative as follows.u+x(x0,t0)=lim
Thus, u_x^+(x_0,t_0)=u_x^-(x_0,t_0):=u_x(x_0,t_0) and \displaystyle\lim_{(x,t)\to(x_0,t_0)}u_x(x,t)=u_x(x_0,t_0). Similarly, we get the partial derivative with repsect to t:\begin{aligned}u_t^+(x_0,t_0)&=\lim_{s\to0^+}\frac{u(x_0,t_0+s)-u(x_0,t_0)}s\\&=\lim_{s\to0^+}\left[\frac{g(x_0+ct_0+cs)-g(x_0+ct_0)}{2s}+\frac{g(-cs)-g(0)}{2s}+\frac1{2cs}\int_{x_0+ct_0}^{x_0+ct_0+cs}\!h(\xi)\,\mathrm d\xi+\frac1{2cs}\int_{-cs}^0\!h(\xi)\,\mathrm d\xi\right]\\&=\frac{c[g'(x_0+ct_0)-g'(0)]}2+\frac{h(x_0+ct_0)+h(0)}2,\\u_t^-(x_0,t_0)&=\lim_{s\to0^-}\frac{u(x_0,t_0+s)-u(x_0,t_0)}s\\&=\lim_{s\to0^-}\left[\frac{\alpha(s)-\alpha(0)}s+\frac{g(ct_0+x_0)-g(ct_0+x_0+cs)}{2s}-\frac{g(cs)-g(0)}{2s}+\frac1{2cs}\int_{ct_0+x_0}^{ct_0+x_0+cs}\!h(\xi)\,\mathrm d\xi-\frac1{2cs}\int_0^{cs}\!h(\xi)\,\mathrm d\xi\right]\\&=\alpha'(0)+\frac{c[g'(ct_0+x_0)-g'(0)]}2+\frac{h(ct_0+x_0)-h(0)}2\\&=\frac{c[g'(ct_0+x_0)-g'(0)]}2+\frac{h(ct_0+x_0)+h(0)}2\quad\text{(by $\alpha'(0)=h(0)$)}.\end{aligned}
Thus, u_t^+(x_0,t_0)=u_t^-(x_0,t_0):=u_t(x_0,t_0) and \displaystyle\lim_{(x,t)\to(x_0,t_0)}u_t(x,t)=u_t(x_0,t_0). For the case that x_0=t_0=0, we can only use the right parital derivatives u_x^+ and u_t^+ to show the continuity of u_x and u_t at the connor. Therefore, u\in C^1(\bar{\mathbb R}_+^2).Now we compute the second order partial derivatives. According to the definition, for the point (x_0,t_0) satisfying x_0=ct_0 for t_0>0, we have
\begin{aligned}u_{xx}^+(x_0,t_0)&=\lim_{s\to0^+}\frac{u_x(x_0+s,t_0)-u_x(x_0,t_0)}s\\&=\lim_{s\to0^+}\left[\frac{g'(x_0+ct_0+s)-g'(x_0+ct_0)}{2s}+\frac{g'(s)-g'(0)}{2s}+\frac{h(x_0+ct_0+s)-h(x_0+ct_0)}{2cs}-\frac{h(s)-h(0)}{2cs}\right]\\&=\frac{g''(x_0+ct_0)+g''(0)}2+\frac{h'(x_0+ct_0)-h'(0)}{2c},\\u_{xx}^-(x_0,t_0)&=\lim_{s\to0^-}\frac{u_x(x_0+s,t_0)-u_x(x_0,t_0)}s\\&=\lim_{s\to0^-}\left[\frac{\alpha'(-s/c)-\alpha'(0)}{-cs}+\frac{g'(ct_0+x_0+s)-g'(ct_0+x_0)}{2s}+\frac{g'(-s)-g'(0)}{2s}+\frac{h(ct_0+x_0+s)-h(ct_0+x_0)}{2cs}+\frac{h(-s)-h(0)}{2cs}\right]\\&=\frac1{c^2}\alpha''(0)+\frac{g''(ct_0+x_0)-g''(0)}2+\frac{h'(ct_0+x_0)-h'(0)}{2c}\\&=\frac{g''(ct_0+x_0)+g''(0)}2+\frac{h'(ct_0+x_0)-h'(0)}{2c}\quad\text{(by $\alpha''(0)=c^2g''(0)$)}.\end{aligned}
Thus, u_{xx}^+(x_0,t_0)=u_{xx}^-(x_0,t_0) and \displaystyle\lim_{(x,t)\to(x_0,t_0)}u_{xx}(x,t)=u_{xx}(x_0,t_0). Similarly, we observe that\begin{aligned}u_{xt}^+(x_0,t_0)&=\lim_{s\to0^+}\frac{u_x(x_0,t_0+s)-u_x(x_0,t_0)}s\\&=\lim_{s\to0^+}\left[\frac{\alpha'(s)-\alpha'(0)}{-cs}+\frac{g'(ct_0+x_0+cs)-g'(ct_0+x_0)}{2s}+\frac{g'(cs)-g'(0)}{2s}+\frac{h(ct_0+x_0+cs)-h(ct_0+x_0)}{2cs}+\frac{h(cs)-h(0)}{2cs}\right]\\&=-\frac1c\alpha''(0)+\frac{c[g''(ct_0+x_0)+g''(0)]}2+\frac{h'(ct_0+x_0)+h'(0)}{2}\\&=\frac{c[g''(ct_0+x_0)-g''(0)]+h'(ct_0+x_0)+h'(0)}2\quad\text{(by $\alpha''(0)=c^2g''(0)$)},\\u_{xt}^-(x_0,t_0)&=\lim_{s\to0^-}\frac{u_x(x_0,t_0+s)-u_x(x_0,t_0)}s\\&=\lim_{s\to0^-}\left[\frac{g'(x_0+ct_0+cs)-g'(x_0+ct_0)}{2s}+\frac{g'(-cs)-g'(0)}{2s}+\frac{h(x_0+ct_0+cs)-h(x_0+ct_0)}{2cs}-\frac{h(-cs)-h(0)}{2cs}\right]\\&=\frac{c[g''(x_0+ct_0)-g''(0)]+h'(x_0+ct_0)+h'(0)}2.\end{aligned}
Thus, u_{xt}^+(x_0,t_0)=u_{xt}^-(x_0,t_0):=u_{xt}(x_0,t_0) and \displaystyle\lim_{(x,t)\to(x_0,t_0)}u_{xt}(x,t)=u_{xt}(x_0,t_0). One may verify that u_{tx}^+(x_0,t_0)=u_{tx}^-(x_0,t_0):=u_{tx}(x_0,t_0), u_{tt}^+(x_0,t_0)=u_{tt}^-(x_0,t_0):=u_{tt}(x_0,t_0), \displaystyle\lim_{(x,t)\to(x_0,t_0)}u_{tx}(x,t)=u_{tx}(x_0,t_0) and \displaystyle\lim_{(x,t)\to(x_0,t_0)}u_{tt}(x,t)=u_{tt}(x_0,t_0). Therefore, u\in C^2(\bar{\mathbb R}_+^2) and we complete the proof.Solution 2
When x\geq ct, the domain of dependence of u(x,t) is the interval [x-ct,x+ct] so we have\displaystyle u(x,t)=\frac{u(x-ct,0)+u(x+ct,0)}2+\frac1{2c}\int_{x-ct}^{x+ct}\!h(s)\,\mathrm ds\quad\text{for}~x\geq ct\geq0.
For 0\leq x<ct, we can employ the parallelogram rule to obtain\begin{aligned}u(x,t)&=u\left(0,t-\frac1cx\right)+u\left(\frac{ct+x}2,\frac{ct+x}{2c}\right)-u\left(\frac{ct-x}2,\frac{ct-x}{2c}\right)\\&=\alpha\left(t-\frac xc\right)+\left[\frac{u(0,0)+u(ct+x,0)}2+\frac1{2c}\int_0^{ct+x}\!h(s)\,\mathrm ds\right]-\left[\frac{u(0,0)+u(ct-x,0)}2+\frac1{2c}\int_0^{ct-x}\!h(s)\,\mathrm ds\right]\\&=\alpha\left(t-\frac xc\right)+\frac{g(ct+x)-g(ct-x)}2+\frac1{2c}\int_{ct-x}^{ct+x}\!h(s)\,\mathrm ds.\end{aligned}
For the case that x_0=t_0=0, we can only use the right partial derivatives u_{xx}^+, u_{xt}^+, u_{tx}^+ and u_{tt}^+ to show the continuity of u_{xx}, u_{xt}, u_{tx} and u_{tt} at the connor. Therefore, we can obtain the same solution as in Solution 1. Following the same argument in Solution 1, u\in C^2(\bar{\mathbb R}_+^2).Solution
Let v(x,t)=u(x,t)+x^2/2 for x\in[0,\pi] and t\in[0,\infty). Then v satisfies\left\{\begin{aligned} &v_{tt}-v_{xx}=0\quad\text{for}~0<x<\pi~\text{and}~t>0,\\&v(x,0)=x^2/2,\quad v_t(x,0)=0\quad\text{for}~0<x<\pi,\\&v(0,t)=0,\quad v(\pi,t)=0\quad\text{for}~t\geq0.\end{aligned}\right.
Using the separation of variables, we suppose that the solution v(x,t) is of the form\displaystyle v(x,t)=\sum_{n=1}^\infty v_n(x,t)=\sum_{n=1}^\infty X_n(x)T_n(t)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0.
Due to the superposition principle, we firstly consider u(x,t)=X(x)T(t) to fulfill the wave equation with the boundary condition, which givesX(x)T''(t)-X''(x)T(t)=0\quad\text{and}\quad X(0)T(t)=X(\pi)T(t)=0.
Surely, we want to find a general solution so T is not identically zero and X(0)=X(\pi)=0. From the differential equation, we have X''(x)/X(x)=T''(t)/T(t) for x\in(0,\pi) and t>0, which means there exists a constant \lambda such that X''(x)=\lambda X(x) and T''(t)=\lambda T(t). To find nontrivial solution X, we divide three cases as follows.\displaystyle v(x,t)=\sum_{n=1}^\infty(c_n\sin(nt)+d_n\cos(nt))\sin(nx)\quad\text{for}~x\in[0,\pi)~\text{and}~t\geq0.
Since v_t(x,0)\equiv0 on (0,\pi), we have\displaystyle0=v_t(x,0)=\sum_{n=1}^\infty nc_n\sin(nx) for x\in(0,\pi),
which gives c_n=0 for n\in\mathbb N. On the other hand, from v(x,0)=x^2/2 on [0,\pi), we have\displaystyle\frac{x^2}2=v(x,0)=\sum_{n=1}^\infty d_n\sin(nx)\quad\text{for}~x\in[0,\pi).
Multiplying it by \sin(mx) and then integrating it over [0,\pi], we obtain\begin{aligned}\frac{(-1)^m(2-m^2\pi^2)-2}{2m^3}&=(-1)^{m+1}\frac{\pi^2}{2m}+\frac{(-1)^m-1}{m^3}\\&=(-1)^{m+1}\frac{\pi^2}{2m}+\left.\frac{x\sin(mx)}{m^2}\right|_0^\pi-\frac1{m^2}\int_0^\pi\!\sin(mx)\,\mathrm dx\\&=\left.-\frac{x^2\cos(mx)}{2m}\right|_0^\pi+\frac1m\int_0^\pi\!x\cos(mx)\,\mathrm dx\\&=\int_0^\pi\!\frac{x^2}2\cdot\sin(mx)\,\mathrm dx=\int_0^\pi\!\sum_{n=1}^\infty d_n\sin(nx)\sin(mx)\,\mathrm dx\\&=d_m\int_0^\pi\!\sin^2(mx)\,\mathrm dx=\frac{d_m}2\int_0^\pi\!(1-\cos(2mx))\,\mathrm dx=\frac\pi2d_m.\end{aligned}
This means \displaystyle d_n=\frac{(-1)^n(2-n^2\pi^2)-2}{n^3\pi} for n\in\mathbb N. Therefore, we obtain\displaystyle v(x,t)=\frac1\pi\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}\cos(nt)\sin(nx)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0~\text{except}~(x,t)=(\pi,0),
which gives the solution\displaystyle u(x,t)=-\frac{x^2}2+\frac1\pi\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}\cos(nt)\sin(nx)\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0~\text{except}~(x,t)=(\pi,0).
The regularity of u is equivalent to that of v because x^2/2 is analytic. Thus, it suffices to investigate the value of v in the various regions bounded by characteristics. To sum the Fourier series of v in regions bounded by characteristics, we define\displaystyle v(x,t)=\frac1{2\pi}\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}[\sin(n(x+t))+\sin(n(x-t))]\quad\text{for}~x\in[0,\pi]~\text{and}~t\geq0~\text{except}~(x,t)=(\pi,0).
From v(x,0)=x^2/2 on [0,\pi), we note that \displaystyle\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}\sin(nx)=\frac{\pi x^2}2 for x\in[0,\pi).In region R_0 (0<x+t<\pi and 0<x-t<\pi), we have
\displaystyle v(x,t)=\frac1{2\pi}\left[\frac{\pi(x+t)^2}2+\frac{\pi(x-t)^2}2\right]=\frac{x^2+t^2}2.
In region R_{3k-2} (0<x+t-(k-1)\pi<\pi and 0<x-t+k\pi<\pi), we have\begin{aligned}v(x,t)&=\frac1{2\pi}\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}[\sin(n(x+t-(k-1)\pi))+\sin(n(x-t+k\pi))]\\&=\frac1{2\pi}\left[\frac{\pi(x+t-(k-1)\pi)^2}2+\frac{\pi(x-t+k\pi)^2}2\right]\\&=\frac{(x+t-(k-1)\pi)^2+(x-t+k\pi)^2}4.\end{aligned}
In region R_{3k-1} (0<x+t-k\pi<\pi and 0<x-t+(k-1)\pi<\pi), we have\begin{aligned}v(x,t)&=\frac1{2\pi}\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}[\sin(n(x+t-k\pi))+\sin(n(x-t+(k-1)\pi))]\\&=\frac1{2\pi}\left[\frac{\pi(x+t-k\pi)^2}2+\frac{\pi(x-t+(k-1)\pi)^2}2\right]\\&=\frac{(x+t-k\pi)^2+(x-t+(k-1)\pi)^2}4.\end{aligned}
In region R_{3k} (0<x+t-k\pi<\pi and 0<x-t+k\pi<\pi), we have\begin{aligned}v(x,t)&=\frac1{2\pi}\sum_{n=1}^\infty\frac{(-1)^n(2-n^2\pi^2)-2}{n^3}[\sin(n(x+t-k\pi))+\sin(n(x-t+k\pi))]\\&=\frac1{2\pi}\left[\frac{\pi(x+t-k\pi)^2}2+\frac{\pi(x-t+k\pi)^2}2\right]\\&=\frac{x^2+(t-k\pi)^2}2.\end{aligned}
It is easy to verify that v is discontinuous along all characteristics, which meansv\in C^\infty\left((0,\pi)\times[0,\infty)-\bigcup_{k=0}^\infty\{(x,t)\in\mathbb R^2\,:\,t\pm x=k\pi\}\right).
Solution
Solution 1
This proof follows from the paper: "Moshinsky's shutter problem: an initial-value problem for the Klein-Gordon equation" (Author: P. A. Martin and F. V. Kowalski). Using the Fourier transform with repsect to x, we define\displaystyle U(\xi,t)=\int_{-\infty}^\infty\!e^{-i\xi x}u(x,t)\,\mathrm dx.
Then U satisfies\begin{aligned}U_{tt}(\xi,t)&=\int_{-\infty}^{\infty}\!e^{-i\xi x}u_{tt}(x,t)\,\mathrm dx=\int_{-\infty}^{\infty}\!e^{-i\xi x}[c^2u_{xx}(x,t)-m^2u(x,t)]\,\mathrm dx\\&=-\xi^2c^2U(\xi,t)-m^2U(\xi,t)=-(\xi^2c^2+m^2)U(\xi,t).\end{aligned}
The initial condition u(x,0)=f(x) and u_t(x,0)=g(x) for x\in\mathbb R can be transformed into\displaystyle U(\xi,0)=F(\xi):=\int_{-\infty}^{\infty}\!e^{-i\xi x}f(x)\,\mathrm dx,\quad U_t(\xi,0)=G(\xi):=\int_{-\infty}^{\infty}\!e^{-i\xi x}g(x)\,\mathrm dx.
We can treat the differential equation of U as ordinary differential equation with respect to t and obtain\displaystyle U(\xi,t)=F(\xi)\cos\left(t\sqrt{\xi^2c^2+m^2}\right)+\frac{G(\xi)}{\sqrt{\xi^2c^2+m^2}}\sin\left(t\sqrt{\xi^2c^2+m^2}\right).
By the inverse Fourier transform, we have\displaystyle u(x,t)=\frac1{2\pi}\int_{-\infty}^{\infty}\!e^{i\xi x}F(\xi)\cos\left(t\sqrt{\xi^2c^2+m^2}\right)\,\mathrm d\xi+\frac1{2\pi}\int_{-\infty}^{\infty}\!e^{i\xi x}\frac{G(\xi)}{\sqrt{\xi^2c^2+m^2}}\sin\left(t\sqrt{\xi^2c^2+m^2}\right)\,\mathrm d\xi.
By (20)-(22) in the reference paper, we have\displaystyle\frac1{2\pi}\int_{-\infty}^{\infty}\!e^{i\xi x}\frac{G(\xi)}{\sqrt{\xi^2c^2+m^2}}\sin(t\sqrt{\xi^2c^2+m^2})\,\mathrm d\xi=\frac1{2c}\int_{-ct}^{ct}\!g(x-y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy,
where J_0 is the Bessel function of first kind of order 0. Moreover, we observe that\begin{aligned}\frac1{2\pi}\int_{-\infty}^{\infty}\!e^{i\xi x}F(\xi)\cos\left(t\sqrt{\xi^2c^2+m^2}\right)\,\mathrm d\xi&=\frac{\partial}{\partial t}\left[\frac1{2\pi}\int_{-\infty}^{\infty}\!e^{i\xi x}\frac{F(\xi)}{\sqrt{\xi^2c^2+m^2}}\sin\left(t\sqrt{\xi^2c^2+m^2}\right)\,\mathrm d\xi\right]\\&=\frac\partial{\partial t}\left[\frac1{2c}\int_{-ct}^{ct}\!f(x-y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy\right]\\&=\frac{f(x-ct)+f(x+ct)}2+\frac{mt}2\int_{-ct}^{ct}\frac{f(x-y)}{\sqrt{c^2t^2-y^2}}J_0'\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy\\&=\frac{f(x-ct)+f(x+ct)}2-\frac{mt}2\int_{-ct}^{ct}\frac{f(x-y)}{\sqrt{c^2t^2-y^2}}J_1\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.\end{aligned}
Here we have used the fact that J_0'(x)=-J_1(x) and J_1 is the Bessel function of first kind of order 1. Therefore, the solution u is given by\displaystyle u(x,t)=\frac{f(x-ct)+f(x+ct)}2-\frac{mt}2\int_{-ct}^{ct}\frac{f(x-y)}{\sqrt{c^2t^2-y^2}}J_1\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy+\frac1{2c}\int_{-ct}^{ct}g(x-y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.
Solution 2
This proof follows from the paper: "Derivation of Solutions of the Klein-Gordon Equation from Solutions of the Wave Equation" (Author: LL. G. Chambers). Due to the linearity of Klein-Gordon equation, we can consider the following equations\left\{\begin{aligned} &v_{tt}-c^2v_{xx}+m^2v=0\quad\text{for}~(x,t)\in\mathbb R\times\mathbb R_+,\\&v(x,0)=f(x),\quad v_t(x,0)=0\quad\text{for}~x\in\mathbb R,\end{aligned}\right.\quad\text{and}\quad\left\{\begin{aligned} &w_{tt}-c^2w_{xx}+m^2w=0\quad\text{for}~(x,t)\in\mathbb R\times\mathbb R_+,\\&w(x,0)=0,\quad w_t(x,0)=g(x)\quad\text{for}~x\in\mathbb R.\end{aligned}\right.
Using the Laplace transform over time, we set\displaystyle V(x,s)=\int_0^\infty\!e^{-st}v(x,t)\,\mathrm dt.
The Laplace transforms of v_{xx} and v_{tt} are\begin{aligned}\int_0^\infty\!e^{-st}v_{xx}(x,t)\,\mathrm dt&=V_{xx}(x,s),\\\int_0^\infty\!e^{-st}v_{tt}(x,t)\,\mathrm dt&=e^{-st}v_t(x,t)\Big|_{t=0}^{t=\infty}+s\int_0^\infty\!e^{-st}v_t(x,t)\,\mathrm dt\\&=s\left[v(x,t)e^{-st}\Big|_{t=0}^{t=\infty}+s\int_0^\infty\!e^{-st}v(x,t)\,\mathrm dt\right]\\&=-sf(x)+s^2V(x,s).\end{aligned}
Here we have used the initial condition v(x,0)=f(x) and v_t(x,0)=0 for x\in\mathbb R. Then we have[s^2V(x,s)-sf(x)]-c^2V_{xx}(x,s)+m^2V(x,s)=0\quad\text{for}~x\in\mathbb R~\text{and}~s>0,
which means(s^2+m^2)V(x,s)-c^2V_{xx}(x,s)=sf(x)\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
Let \tilde v be the solution of\left\{\begin{aligned} &\tilde v_{tt}-c^2\tilde v_{xx}=0\quad\text{for}~x\in\mathbb R~\text{and}~t>0,\\&\tilde v(x,0)=f(x),\quad\tilde v_t(x,0)=0\quad\text{for}~x\in\mathbb R.\end{aligned}\right.
Set \displaystyle\tilde V(x,s)=\int_0^\infty\!e^{-st}\tilde v(x,t)\,\mathrm dt. Then the wave equation with initial condition can be transformed into[s^2\tilde V(x,s)-sf(x)]-c^2\tilde V_{xx}(x,s)=0\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
Replacing s with \sqrt{s^2+m^2}, we get(s^2+m^2)\tilde V(x,\sqrt{s^2+m^2})-c^2\tilde V_{xx}(x,\sqrt{s^2+m^2})=\sqrt{s^2+m^2}f(x)\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
Moreover, we may observe that\displaystyle(s^2+m^2)\left[\frac{s\tilde V(x,\sqrt{s^2+m^2})}{\sqrt{s^2+m^2}}\right]-c^2\left[\frac{s\tilde V(x,\sqrt{s^2+m^2})}{\sqrt{s^2+m^2}}\right]_{xx}=sf(x).
Thus, we arrive at\displaystyle V(x,s)=\frac{s}{\sqrt{s^2+m^2}}\tilde V(x,\sqrt{s^2+m^2})\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
To apply the inverse Laplace transform to V with respect to s, we need to know the inverse Laplace transform of \alpha(s):=s/\sqrt{s^2+m^2} and \beta(s):=\tilde V(x,\sqrt{s^2+m^2}). Suppose that the inverse Laplace transform of \alpha(s) and \beta(x,s) can be denoted by A(t) and B(x,t), respectively. Then by conventional theorem, we have\displaystyle v(x,t)=\int_0^t\!A(\tau)B(x,t-\tau)\,\mathrm d\tau.
By the formula of inverse Laplace transform, one may get\begin{equation}\label{1}B(x,t)=\tilde v(x,t)-m\int_0^t\!\tilde v(x,\sqrt{t^2-\xi^2})J_1(m\xi)\,\mathrm d\xi,\end{equation}where J_1 is the Bessel function of the first kind of order 1. This formula for m=1 can be found at page 123 in the German book (Formeln und Sätze für die Speziellen Funktionen der Mathematischen Physik). However it seems extermely difficult to derive it directly. Here we may simplify the expression of B by using \tilde v(x,t)=\frac{f(x-ct)+f(x+ct)}2 and the substitution for y=c\sqrt{t^2-\xi^2}:\begin{aligned}B(x,t)&=\frac{f(x-ct)+f(x+ct)}2-mc\int_0^{ct}\frac{y[f(x-y)+f(x+y)]}{2\sqrt{c^2t^2-y^2}}J_1\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy\\&=\frac{f(x-ct)+f(x+ct)}2-\frac{mc}2\int_{-ct}^{ct}\frac{yf(x-y)}{\sqrt{c^2t^2-y^2}}J_1\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.\end{aligned}
On the other hand, we can follow the derivation from paper or verify it to getA(t)=mJ_0'(mt)+\delta(t)=-mJ_1(mt)+\delta(t),
where J_0 is the Bessel function of first kind of order 0 and \delta is the Dirac delta functional. Thus, we obtain\begin{aligned}v(x,t)&=B(x,t)-m\int_0^tJ_1(m\tau)B(x,t-\tau)\,\mathrm d\tau.\end{aligned}
It may become\displaystyle v(x,t)=\frac{f(x-ct)+f(x+ct)}2-\frac{mt}2\int_{-ct}^{ct}\frac{f(x-y)}{\sqrt{c^2t^2-y^2}}J_1\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.
To solve w, we use the Laplace transform over time again to set\displaystyle W(x,s)=\int_0^\infty\!e^{-st}w(x,t)\,\mathrm dt.
The Laplace transforms of w_{xx} and w_{tt} are\begin{aligned}\int_0^\infty\!e^{-st}w_{xx}(x,t)\,\mathrm dt&=W_{xx}(x,s),\\\int_0^\infty\!e^{-st}w_{tt}(x,t)\,\mathrm dt&=e^{-st}w_t(x,t)\Big|_{t=0}^{t=\infty}+s\int_0^\infty\!e^{-st}w_t(x,t)\,\mathrm dt\\&=-g(x)+s\left[e^{-st}w(x,t)\Big|_{t=0}^{t=\infty}+s\int_0^\infty\!e^{-st}w(x,t)\,\mathrm dt\right]\\&=-g(x)+s^2W(x,s).\end{aligned}
Here we have used the initial condition w(x,0)=0 and w_t(x,0)=g(x) for x\in\mathbb R. Then we have(s^2+m^2)W(x,s)-c^2W_{xx}(x,s)=g(x)\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
Let \tilde w be the solution of\tilde w_{tt}-c^2\tilde w_{xx}=0\quad\text{for}~x\in\mathbb R~\text{and}~t>0,\\\tilde w(x,0)=0,\quad\tilde w_t(x,0)=g(x)\quad\text{for}~x\in\mathbb R.
Set \displaystyle\tilde W(x,s)=\int_0^\infty\!e^{-st}\tilde w(x,t)\,\mathrm dt. Then the wave equation with initial condition can be transformed intos^2\tilde W(x,s)-c^2\tilde W_{xx}(x,s)=g(x)\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
Replacing s with \sqrt{s^2+m^2}, we may observe thatW(x,s)=\tilde W(x,\sqrt{s^2+m^2})\quad\text{for}~x\in\mathbb R~\text{and}~s>0.
Using the same formula as \eqref{1}, we have\begin{aligned}w(x,t)&=\tilde w(x,t)-m\int_0^t\!\tilde w\left(x,\sqrt{t^2-\xi^2}\right)J_1(m\xi)\,\mathrm d\xi\\&=\tilde w(x,t)+m\int_0^t\tilde w\left(x,\sqrt{t^2-\xi^2}\right)J_0'(m\xi)\,\mathrm d\xi\\&=\tilde w(x,t)+\left.J_0(m\xi)\tilde w\left(x,\sqrt{t^2-\xi^2}\right)\right|_{\xi=0}^{\xi=t}+\int_0^t\!\frac{\xi\tilde w_t\left(x,\sqrt{t^2-\xi^2}\right)}{\sqrt{t^2-\xi^2}}J_0(m\xi)\,\mathrm d\xi\\&=\int_0^t\!\frac{\xi\tilde w_t\left(x,\sqrt{t^2-\xi^2}\right)}{\sqrt{t^2-\xi^2}}J_0(m\xi)\,\mathrm d\xi\quad\text{(by $J_0(0)=1$ and $\tilde w(x,0)=0$)}\\&=-\frac1c\int_{ct}^0\!\tilde w_t(x,y/c)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy\quad\text{(by $y=c\sqrt{t^2-\xi^2}$)}\\&=\frac1{2c}\int_0^{ct}\![g(x+y)+g(x-y)]J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy\quad\text{(by $\displaystyle w_t(x,t)=\frac{g(x+ct)+g(x-ct)}2$)}\\&=\frac1{2c}\int_{-ct}^{ct}\!g(x-y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.\end{aligned}
Here the last equality is used the fact that\displaystyle\int_0^{ct}\!g(x+y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy=\int_{-ct}^0\!g(x-y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.
Finally, we obtain the solution u is\displaystyle u(x,t)=B(x,t)+\tilde w(x,t)-m\int_0^tJ_1(m\tau)B(x,t-\tau)\,\mathrm d\tau-m\int_0^t\!\tilde w(x,\sqrt{t^2-\xi^2})J_1(m\xi)\,\mathrm d\xi.
It could become\displaystyle u(x,t)=\frac{f(x-ct)+f(x+ct)}2-\frac{mt}2\int_{-ct}^{ct}\frac{f(x-y)}{\sqrt{c^2t^2-y^2}}J_1\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy+\frac1{2c}\int_{-ct}^{ct}g(x-y)J_0\left(m\sqrt{t^2-\frac{y^2}{c^2}}\right)\,\mathrm dy.
Solution 3
The idea comes from the Exercise 5 of Section 3.2. Let v(x,y,t)=\cos(my/c)u(x,t) for (x,y,t)\in\mathbb R^2\times\bar{\mathbb R}_+. Then it is easy to find that\begin{aligned}v_{tt}(x,y,t)&=\cos(my/c)u_{tt}(x,t)=\cos(my/c)[c^2u_{xx}(x,t)-m^2u(x,t)]\\&=c^2v_{xx}(x,y,t)+c^2v_{yy}(x,y).\end{aligned}
This means v satisfies the two-dimensional wave equation with the initial conditionv(x,y,0)=\cos(my/c)u(x,0)=f(x)\cos(my/c),\quad v_t(x,y,0)=\cos(my/c)u_t(x,0)=g(x)\cos(my/c)\quad\text{for}~(x,y)\in\mathbb R^2.
Then by the formula (39) in Section 3.2, we obtain\begin{aligned}v(x,y,t)&=\frac1{2\pi}\frac\partial{\partial t}\left(t\int_{\xi_1^2+\xi_2^2<1}\frac{f(x+ct\xi_1)\cos(m(y+ct\xi_2)/c)}{\sqrt{1-\xi_1^2-\xi_2^2}}\,\mathrm d\xi_1\,\mathrm d\xi_2\right)\\&\quad+\frac t{2\pi}\int_{\xi_1^2+\xi_2^2<1}\frac{g(x+ct\xi_1)\cos(m(y+ct\xi_2)/c)}{\sqrt{1-\xi_1^2-\xi_2^2}}\,\mathrm d\xi_1\,\mathrm d\xi_2.\end{aligned}
Hence the solution u is\begin{aligned}u(x,t)&=v(x,0,t)\\&=\frac1{2\pi}\frac\partial{\partial t}\left(t\int_{\xi_1^2+\xi_2^2<1}\frac{f(x+ct\xi_1)\cos(mt\xi_2)}{\sqrt{1-\xi_1^2-\xi_2^2}}\,\mathrm d\xi_1\,\mathrm d\xi_2\right)\\&\quad+\frac t{2\pi}\int_{\xi_1^2+\xi_2^2<1}\frac{g(x+ct\xi_1)\cos(mt\xi_2)}{\sqrt{1-\xi_1^2-\xi_2^2}}\,\mathrm d\xi_1\,\mathrm d\xi_2\\&=\frac1{2\pi}\frac\partial{\partial t}\left(t\int_0^{2\pi}\!\int_0^1\!\frac{r\cos\left(mtr\sin\theta\right)f(x+ctr\cos\theta)}{\sqrt{1-r^2}}\,\mathrm dr\,\mathrm d\theta\right)\\&\quad+\frac t{2\pi}\int_0^{2\pi}\!\int_0^1\!\frac{r\cos(mtr\sin\theta)g(x+ctr\cos\theta)}{\sqrt{1-r^2}}\,\mathrm dr\,\mathrm d\theta.\end{aligned}
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