Classification of PDEs?

[pivoxa15]

1. The problem statement, all variables and given/known data
If you are presented with a PDE with a d(u)/dt in it, how would you classify it?

There is not t dependence in the classification section of PDEs

http://en.wikipedia.org/wiki/Partial_differential_equation

[pivoxa15]

Is it the case that if you have a t depdence, and one of x or y then replace one of x or y with t and use the master 2 variable 2nd order PDE form. However if you had x, y and t in a PDE than that is a 3 variable PDE and would be different. Why do they only consider PDEs with only 2 variables? Are there classifications for 3 or more variables? Or is it the case that if you want to extend the 2 variables case to more you could use vector calculus and replace the x by (x,y,z) and have the t there so 4 variables.

[HallsofIvy]

Surely you understand that the classification of P.D.E.s does not depend on what you happen to CALL a variable. If one of the independent variables occurs in a first but not second derivative (and there is at least one other independent variable with a second derivative) that is a parabolic equation.

In particular, the "diffusion" or "heat" equation
\frac{\partial^2 u}{\partial x^2}= \kappa \frac{\partial u}{\partial t} is parabolic.

[pivoxa15]

I'm new to PDEs and I think they are exponentially harder than ODEs. There is so much behind an innocent looking PDE like the over you describe above. Is that why there have been fields medals awarded for people who have produced results in PDE theory.