In my research work, I am dealing with a quasi-linear system of first order p.d.e.'s with two independent variables (say $x\_1$ and $x\_2$) and four dependent variables (say $u\_1(x\_1,x\_2)$, $u\_2(x\_1,x\_2)$, $u\_3(x\_1,x\_2)$ and $u\_4(x\_1,x\_2)$) of the form
\begin{equation}
\mathbf{A}(\mathbf{x},\mathbf{u}(\mathbf{x}))\frac{\partial \mathbf{u}}{\partial x\_1}+\mathbf{B}(\mathbf{x},\mathbf{u}(\mathbf{x}))\frac{\partial \mathbf{u}}{\partial x\_2}=\mathbf{f}(\mathbf{x}),
\end{equation}
where $\mathbf{x}=[x\_1\,\,x\_2]^{T}$ and $\mathbf{u}=[u\_1\,\,u\_2\,u\_3\,u\_4]^{T}$ are, respectively, the independent and dependent vector variables, constructed out of $x\_\alpha$'s and $u\_i$'s, $\alpha=1,2$ and $i=1,2,3,4$. $\mathbf{A}$ and $\mathbf{B}$ are two $4\times 4$ real matrices which are functions of $\mathbf{x}$ and $\mathbf{u}$ (dependence on $\mathbf{u}$ makes the system quasi-linear) and $\mathbf{f}$ is a known vector valued function of $\mathbf{x}$.


To determine the type of the system, we consider the eigenvalues of the matrix $\mathbf{A}^{-1}\mathbf{B}$ (assuming $\mathbf{A}$ to be invertible which it is in my case). Now there are 4 possibilities about the nature of these eigenvalues and there are corresponding "type" of the p.d.e. system defined:


1. All eigenvalues real and distinct, implying hyperbolic system. We have the well-studied system of hyperbolic system of conservation laws falling under this category.
2. All eigenvalues are complex (two pairs of complex conjugate eigenvalues). We have then an elliptic system. (I don't know about any physical example of this type.)
3. All eigenvalues are real but with unequal algebraic and geometric multiplicity (i.e. no set of 4 linearly independent eingenvectors). We then have a parabolic system. (Again, no knowledge of physical example.)
4. Two real and one pair of complex conjugate eigenvalues. This does not fall into the above categories. Probably these are called the quasi-linear system of mixed type.


My first question is about the the above classification scheme which, in fact, imitates the classification scheme of linear system of first order p.d.e.'s. Matrices $\mathbf{A}$ and $\mathbf{B}$ involve unknown function $\mathbf{u}$. So, how to determine the type of the system (from the nature of its eigenvalues) a priori to the determination of the solution?


My second question is about the type 4 systems, because my problem falls into this category. How to deal with this mixed type p.d.e. systems?


My major is mechanical engineering and I am working in theory of elasticity. Any help would be greatly appreciated as I know very little about analytical methods to solve quasi-linear p.d.e. systems of first order.


Just to mention, in a previous question I was concerned about a system of 3 linear first order pdes of mixed type (3 roots of the characteristic polynomial: either all are real, or one real and the others appearing as a complex conjugate pair) which is a special case of this problem. To emphasize, I come up with these systems in a physical problem and I know that there must exist real valued "well-behaved" solutions (bounded). Which I do not know is how to find those solutions which must exist and I am looking for possible methods in this forum.


Thanks in advance for any help.



Quasi-linear System of First Order P.D.E.s of "Mixed" type
Let me change your notations. You deal with a 1D quasilinear system with size $N=4$: the standard Cauchy problem is
$$
\frac{\partial u}{\partial t}+A(t,x,u)\frac{\partial u}{\partial x}= f(t,x),\quad u(t=0,x)=u\_0(x),
$$
where $t\in \mathbb R$ (time variable) as well as $x$ (this is a 1D problem), $u$ is valued in $\mathbb R^N$, $A$ is a real-valued $N\times N$ matrix. You may also assume that $A$ depends smoothly (or even analytically) of its arguments.


(1) Your first case: all eigenvalues of $A\_0:=A(0,x\_0, u\_0(x\_0))$ are real and distinct, this is indeed the strictly hyperbolic case. In this case, you can guarantee local existence, uniqueness and continuous dependence on the data, i.e. local well-posedness. Of course you cannot expect global existence in general because of the nonlinearity (think about the scalar Burgers).


(2) Let me skip some of your cases and go directly to the case where $A\_0$ has a non-real eigenvalue (and thus a pair of non-real eigenvalues). Big trouble ahead: even if the matrix $A$ is analytic, in which case, Cauchy-Kovalevskaya theorem is providing a local (unique) analytic solution, that solution is very unstable in the Hadamard sense. It means that even though $v\_0-u\_0$ is very small in a very strong topology, such as the $C^\infty$ topology or the $H^s$ topology for a very large $s$, you will not be able to control $u(t)-v(t)$ in a quite weak topology such as $L^2$ (all this is local of course). You will find precise statements in a paper by Métivier, Remarks on the well-posedness of the nonlinear Cauchy
problem, with MR number MR2127041.


(3) When all eigenvalues are real, some with multiplicity larger than 2, then instability could or could not occur, depending on other structural factors such as semi-simplicity of $A\_0$. Generally speaking, multiple roots will trigger difficulties.


(4) A very important class of theorems, with the name of Lax-Mizohata theorems is establishing a weak converse to (1): if the problem is well-posed (e.g. meaning that you have some Sobolev norm control of $u(t)$ by some Sobolev norm of $u(0)$), then it implies that the system is 
weakly hyperbolic (case (3)$\cup$(1)). So if you expect your system to be well-behaved in the sense of Hadamard, no choice, the roots must be real-valued, possibly with multiplicity.


Do not think that all physically relevant problem of that type are hyperbolic: to quote just one example, Van der Waals classical system is
$$
\partial\_t u+\partial\_x v=0\quad \partial\_t v+\partial\_x q(u)=0.
$$
When $q'(u)>0$, you are in a hyperbolic region, but when $q'(u)<0$, you have a non-real eigenvalue.


