[Differential Equations II] Final Report

In this report, we study the global existence of solutions to nonlinear dispersive wave equations \begin{eqnarray*}\partial_t^2 u + \frac{1}{\rho^2} |\partial_x|^{2\rho} u &=& \lambda |\partial_t u |^{p-1} \partial_t u\end{eqnarray*} in one space dimension where $0 < \rho \leqslant 2$, $\rho \neq … Continue reading

[M1 Seminar II] Week 8 : Lorentz Invariance of the Wave Equations

Now we will introduce some lemmas for the proof of the global existence theorem for the nonlinear wave equations with quadratic nonlinearities. The exsistence theorem which was proven before gives $n >5$ for the quadratic nonlinearities. But in fact this … Continue reading

[M1 Seminar II] Week 7 : A Global Existence Theorem for Nonlinear Wave Equations

We consider the following initial value problem \[ y_{tt} – \Delta y = f(Dy,\nabla Dy)\;\;\;\;\text{with}\;\;\;\; y(t=0) = y_0,\; y_t (t=0) = y_1 \tag{P} \] with $f \in C^\infty ( \Bbb R^{(n+1)^2}, \Bbb R)$, $\exists \alpha \in \Bbb N$ such that … Continue reading

편미분 방정식 전공 서적 리뷰 (PDE, Partial Differential Equations Book)

이 쯤에서 그간 읽거나 접했던 PDE 전공서적을 리뷰 해 볼까 합니다. 아직 책을 평가할 수준이 절대로 안 되기 때문에 나름대로의 감상평에 불과합니다.   1. F. John. Partial Differential Equations. Springer. Courant의 제자 F. John이 쓴 책입니다. PDE 입문용으로 적당한 것 … Continue reading

[M1 Seminar II] Week 6 : Weighted a priori Estimates for Small Data

For the proof of the global existence theorem of nonlinear wave equations, this a priori estimates is essential as well as the high energy estimates. Also the high energy estimates play an important role in the proof of a priori … Continue reading

[M1 Seminar II] Week 5 : High Energy Estimates

We prove the high energy estimates for the nonlinear wave equation on the nonlinearity $\alpha =1$. In fact, this problem is a special case of the quasi-linear symmetric hyperbolic system with some assumptions. By the help of the previous existence … Continue reading

[M1 Seminar II] Week 4 : Regularities of the Solution and and Improved Existence Theorem

We proved the right-continuity of the solution at the initial time in $W^{s,2}$ which implies the continuity of the whole interval. And by the PDE and Sobolev embedding theorem, we can easily get the regularity in the existence theorem. Also … Continue reading

[M1 Seminar II] Week 3 : Regularities of the Solution

This week we will prove that the solution of the system $u$ is in $C^0([0,T], W^{s,2}) \cap C^1([0,T], W^{s-1,2})$ so that $u \in C_b^1 ([0,T] \times \Bbb R^n)$. For this purpose we first show that $u \in L^\infty ([0,T], W^{s,2})$, … Continue reading

[M1 Seminar II] Week 2 : Local Existence for Quasi-linear Symmetric Hyperbolic Systems (2)

This week, we will prove the local existence of quasi-linear symmetric hyperbolic systems by using $u^{k}$ which is iteratively defined by the solution of the linear symmetric hyperbolic system. For this purpose we first proved the boundedness of $u^k$ in … Continue reading

[M1 Seminar] Week 13 : Elementary Inequalities on the Klainerman Vector Field

We construct the Klainerman vector field with some operators which were defined in the last seminar. And we prove some basic inequalities of these family of operators which play an important role for our final goal. M1_Semi_Week13  

[M1 Seminar] Week 11,12 : Introduction to the Klainerman Vector Field

We introduce the Klainerman vector field and prove some commutator relations which will play an important role in the proof of the global existence theorem for the nonlinear wave equations. M1_Semi_Week11-12  

[M1 Seminar] Week 6,7 : A Global Existence Theorem to the Linear Symmetric Hyperbolic Systems

This week, we prove the global existence theorem to the linear symmetric hyperbolic system by using the standard energy inequality. M1_Semi_Week6-7  

[M1 Seminar] Week 5 : Energy Estimates for the Linear Symmetric Hyperbolic System

We derive the corresponding estimates for higher derivatives from the basic energy estimates which was given in the last week. We also prove a simple corollary using this result. And we discuss about the finite propagation speed of the hyperbolic … Continue reading

[M1 Seminar] Week 2 : Decay Estimates for Linear Wave Equations

   This week, we have considered only odd space dimensions for the linear wave equation to prove two estimates in Theorem 2.1. The proof was done for the simple case. For the general cases including the case of even dimensions, we … Continue reading