# Introduction to Mathematical Physics/Some mathematical problems and their solution/Nonlinear evolution problems, perturbative methods
## Problem statement

Perturbative methods allow to solve nonlinear evolution problems. They are used in hydrodynamics, plasma physics for solving nonlinear fluid models (see for instance ([ph:plasm:Chen84]). Problems of nonlinear ordinary differential equations can also be solved by perturbative methods (see for instance ([ma:equad:Arnold83]) where averaging method is presented).
Famous KAM theorem (Kolmogorov--Arnold--Moser) gives important results about the perturbation of hamiltonian systems. Perturbative methods are only one of the possible methods: geometrical methods, normal form methods ([ma:equad:Arnold83]) can give good results. Numerical technics will be introduced at next section.
Consider the following problem:
proeqp
Problem: Find $u\in V$ such that:
${\frac {\partial u}{\partial t}}=Lu+N(u),u\in E,x\in \Omega$
* $u$ verifies boundary conditions on the border $\partial \Omega$ of $\Omega$.
* $u$ verifies initial conditions.
Various perturbative methods are presented now.

## Regular perturbation

Solving method can be described as follows:
Algorithm:
* Differential equation is written as:
${\frac {\partial u}{\partial t}}=Lu+\epsilon N(u)$
* The solution $u_{0}$ of the problem when $\epsilon$ is zero is known.
* General solution is sought as:
$u(t)=\sum \epsilon ^{i}u_{i}(t)$
* Function $N(u)$ is developed around $u_{0}$ using Taylor type formula:
$N(u_{0}+\epsilon u_{1})=N(u_{0})+\epsilon u_{1}\left.{\frac {\partial N}{\partial u}}\right)_{u_{0}}$
* A hierarchy of linear equations to solve is obtained:
${\frac {\partial u_{0}}{\partial t}}=Lu_{0}$
${\frac {\partial u_{1}}{\partial t}}=Lu_{1}+u_{1}\left.{\frac {\partial N}{\partial u}}\right)_{u_{0}}$
This method is simple but singular problem my arise for which solution is not valid uniformly in $t$.
Example:
Non uniformity of regular perturbative expansions (see ([ma:equad:Bender87]). Consider Duffing equation:
${\frac {d^{2}y}{dt^{2}}}+y+\epsilon y^{3}=0$
Let us look for solution $y(t)$ which can be written as:
$y(t)=y_{0}(t)+\epsilon y_{1}(t)+\epsilon ^{2}y_{2}(t)$
The linear hierarchy obtained with the previous assumption is:
${\begin{matrix}{\frac {d^{2}y_{0}}{dt^{2}}}+y_{0}&=&0\\{\frac {d^{2}y_{1}}{dt^{2}}}+y_{1}&=&-y_{0}^{3}\end{matrix}}$
With initial conditions:
$y_{0}(0)=1,y'_{0}(0)=0$,
one gets:
$y_{0}(t)=cos(t)$
and a particular solution for $y_{1}$ will be unbounded , now solution is expected to be bounded. Indeed (see [ma:equad:Bender87]), multiplying Duffing equation by ${\dot {y}}$, one gets the following differential equation:
${\frac {d}{dt}}[{\frac {1}{2}}({\frac {dy}{dt}})^{2}+{\frac {1}{2}}y^{2}+{\frac {1}{4}}\epsilon y^{4}]=0.$
We have thus:
${\frac {1}{2}}({\frac {dy}{dt}})^{2}+{\frac {1}{2}}y^{2}+{\frac {1}{4}}\epsilon y^{4}=C$
where $C$ is a constant. Thus
$y^{2}$ is bounded if $\epsilon >0$.
Remark:
In fact Duffing system is conservative.
Remark:
Origin of secular terms : A regular perturbative expansion of a periodical function whose period depends on a parameter gives rise automatically to secular terms (see ([ma:equad:Bender87]):
$\sin((1+\epsilon )t)=\sin(t)cos(\epsilon t)+\sin(\epsilon ).cos(t)$
$=\sin(t)(1+{\frac {\epsilon ^{2}t^{2}}{2}}+\dots )+(\epsilon t+\dots ).cos(t)$

## Born's iterative method

Algorithm:
* Differential equation is transformed into an integral equation:
$u=\int _{0}^{t}(Lu+N(u))dt'$
* A sequence of functions $u_{n}$ converging to the solution $u$ is sought:
Starting from chosen solution $u_{0}$, successive $u_{n}$ are evaluated using recurrence formula:
$u_{n+1}=\int _{0}^{t}(Lu_{n}+N(u_{n}))dt'$
This method is more "global" than previous one \index{Born iterative method} and can thus suppress some divergencies. It is used in diffusion problems ([ph:mecaq:Cohen73],[ph:mecaq:Cohen88]). It has the drawback to allow less control on approximations.

## Multiple scales method

Algorithm:
* Assume the system can be written as: eqavece ${\frac {\partial u}{\partial t}}=Lu+\epsilon N(u)$
* Solution $u$ is looked for as: eqdevmu $u(x,t)=u_{0}(x,T_{0},T_{1},\dots ,T_{N})+\epsilon u(x,T_{0},T_{1},\dots ,T_{N})+\dots +O(\epsilon ^{N})$ with $T_{n}=\epsilon ^{n}t$ for all $n\in \{0,\dots ,N\}$.
* A hierarchy of equations to solve is obtained by substituting expansion eqdevmu into equation eqavece.
For examples see ([ma:equad:Nayfeh95]).

## Poincaré-Lindstedt method

This method is closely related to previous one, but is specially dedicated to studying periodical solutions. Problem to solve should be:\index{Poincaré-Lindstedt}
Problem:
Find $u$ such that:
eqarespo
$G(u,\omega )=0$
where $u$ is a periodic function of pulsation $\omega$. Setting $\tau =\omega t$,
one gets:
$u(x,\tau +2\pi )=u(x,\tau )$
Resolution method is the following:
Algorithm:
* Existence of a solution $u_{0}(x)$ which does not depend on $\tau$ is imposed: fix $G(u_{0}(x),\omega )=0$
* Solutions are sought as: form1 $u(x,\tau ,\epsilon )=u_{0}(x)+\epsilon u_{1}(x,\tau )+{\frac {\epsilon ^{2}}{2}}u_{2}(x,\tau )+\dots$ form2 $\omega (\epsilon )=\omega _{0}+\epsilon \omega _{1}+{\frac {\epsilon }{2}}\omega _{2}$ with $u(x,\tau ,\epsilon =0)=u_{0}(x)$.
* A hierarchy of linear equations to solve is obtained by expending $G$ around $u_{0}$and substituting form1 and form2 into eqarespo.

## WKB method

mathsecWKB
WKB (Wentzel-Krammers-Brillouin) method is also a perturbation method. It will be presented at section secWKB in the proof of ikonal equation.
* ↑ Indeed solution of equation: ${\ddot {y}}+y=\cos t$ is $y(t)=Acost+B\sin t+{\frac {1}{2}}t\sin t$