# Classification of Second-Order PDE with Constant Coefficients

## Homework Statement

I have 3 equations:

$\frac{\partial^2 u}{\partial t^2}+\frac{\partial^2 u}{\partial x \partial t}+\frac{\partial^2 u}{\partial x^2}$

$\frac{\partial^2 u}{\partial t^2}+4\frac{\partial^2 u}{\partial x \partial t}+4\frac{\partial^2 u}{\partial x^2}$

$\frac{\partial^2 u}{\partial t^2}-4\frac{\partial^2 u}{\partial x \partial t}+\frac{\partial^2 u}{\partial x^2}$

I know their classification (first one is elliptical, second one is parabolic, third one is hyperbolic).
I need to find their characteristics. How do I do that?
What is a characteristic?
I think that the one who is elliptical has no characteristic, the parabolic has only one and the hyperbolic has two.

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HallsofIvy
Homework Helper
Yes, that is true. But how could you know that (or what it means) if you don't know what a "characteristic" is?
A characteristic is a curve upon which the partial differential equation separates into two ordinary differential equations. For example, if I associate the last formula (These are NOT equations because there is no "=". Did you mean "= 0"?) with "$T^2- 4TX+ X^2$ then by "completing the square" I get $T^2- 4TX+ 4X^2- 3X^2= (T- 2X)^2- (\sqrt{3}X)^2= (T- 2X+\sqrt{3}X)(T- 2X- \sqrt{3}X)$

So the "characteristics" are the curves $t- (2-\sqrt{3})x= C$ and $t- (2+ \sqrt{3})x= C$ for C any constant.

Similarly, for the parabolic equation, we can write $T^2+ 4TX+ 4X^2=N (T+ 2X)^2$ and so have the single characteristic $t+ 2x= C$.

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