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Partial differential equations (PDEs): Existence and uniqueness, regularity, boundary conditions, linear and non-linear operators, stability, soliton theory, integrable PDEs, conservation laws, qualitative dynamics.

12 votes
Accepted

Theorems that tell if an explicit analytical solution is possible for nonlinear PDEs

As far as (local) power series solutions go (i.e., in the analytic category) the main existence theorem is the Cauchy-Kowalewski Theorem (in the determined, non characteristic case) and its generaliza …
Robert Bryant's user avatar
3 votes
Accepted

Proving compatibility of two Partial differential equations

Actually, you have described three entirely different notions of 'compatibility' of a pair of first order PDE for a single function of two variables. The first two that you have listed are not the st …
Robert Bryant's user avatar
7 votes
Accepted

Variational formulation of second order equations of the divergence form

Actually, Math604 is closer to the right answer. To see the correct condition, you need to pay attention to the placement of your indices. Your operator should be written in the form $$ Lu = \partia …
Robert Bryant's user avatar
4 votes

Classification of a certain System of Linear First Order PDEs whose characteristic polynomia...

The following device might help in applying Deane's suggestions. At least the calculations on the leading order terms will be simpler: Consider the change of variables $$ \begin{align} v &= u^1 + u^ …
Robert Bryant's user avatar
5 votes
Accepted

Integrability conditions for 'componentwise' systems of linear PDEs

The answer is, in general, yes. There are further integrability conditions. What you should be considering is the system of $1$-forms on $\mathbb{R}^n\times \mathbb{R}\times \mathbb{R}^n$ (with coor …
Robert Bryant's user avatar
10 votes
Accepted

Solving PDE with Cauchy - Kowalewski Theorem

You don't need the Cauchy-Kowalewski Theorem for your problem. In fact, real-analyticity is a red herring here. What you are asking for is a function $\beta(x,y)$ such that the graph $\bigl(x,y,\bet …
Robert Bryant's user avatar
7 votes

Solving a simple PDE on a 2 dimensional manifold

Away from the zeros of $X$, it's always locally solvable. Just put $X$ in flowbox form, i.e., $X =\partial_x$. Write $Y = u\ \partial_x + v\ \partial_y$, and the equation uncouples into the pair of …
Robert Bryant's user avatar
6 votes
Accepted

Using method of characteristics to solve a system of first order quasilinear PDEs

The method of characteristics will tell you that, for any given solution $\bigl(f(x,t),g(x,t)\bigr)$, the characteristic curves are given by the foliations $$ \mathrm{d}x + \bigl(3f(x,t)^2{+}2g(x,t)\b …
Robert Bryant's user avatar
12 votes
Accepted

Pde system problem

I assume that, in the surface case, the OP wants to interpret $S$ as a surface endowed with a Riemannian metric and wants to understand the solutions to the equations $\Delta f - hf^2 = 0$ and $|\nabl …
Robert Bryant's user avatar
13 votes

Special Second-Order PDE

This is not really an answer, just a sequence of comments that are all related, but are too long to put into a comment field. First, some good news: When $n=1$, there's always a (unique) solution f …
Robert Bryant's user avatar
4 votes
Accepted

Existence and uniqueness of a quasi-linear pde system on a surface

This is not a full solution, but it indicates the main points. (A full solution would involve an EDS analysis that would be too long to include here; see below.) First, setting $I_\alpha = (\Delta + …
Robert Bryant's user avatar
2 votes

How to Separate Charpit Equations

N.B.: I checked your calculations a couple of times, and I got a different sign in the Charpit equations: $$ -\frac{x}{N^2}dp=-\frac{1}{x^2 y p}dx=\frac{1}{2\Omega x^2 y^2} dy $$ (Notice the minus si …
Robert Bryant's user avatar
4 votes
Accepted

On the solution of a Monge-Ampere type non-linear partial differential equation

When you write the solution, you must have some other conditions in mind, since one generally does not have unique solutions. For example $\zeta = 0$ and $\zeta = 2 w$ both satisfy your equation, so …
Robert Bryant's user avatar
6 votes
Accepted

For an arbitrary $G(x,t)$, does $f_t=2G_xf+Gf_x$, $f(x,0)=0$ have a unique solution for $f$?

The answer is "No, it is not necessarily true that $f(x,t)=0$ for all $(x,t)\in\mathbb{R}^2$". Here is a counterexample: Let $G(x,t) = x^2$ and let $h:\mathbb{R}\to\mathbb{R}$ be any smooth function …
Robert Bryant's user avatar
6 votes
Accepted

Does $\partial\overline{\partial}f=0$ imply $f\equiv c$ for particular kind of $f$?

Yes, this is true. The point is that $\partial\bar\partial f = 0$ on the complement $C$ of a ball in $\mathbb{C}^n$ ($n\ge 2$) implies that $f = h_+ + \overline{h_-}$ for some functions $h_\pm$ that …
Robert Bryant's user avatar

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