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\begin{equation}
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\pdvt{u^I} + \tensor{A}{^i^I_J} \pdv{u^J}{x^i} = S^I,
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\end{equation}
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where both $A$ and $S$ can depend on $u^I$ but not their derivatives. For any unit covector $\xi_i$, the \emph{Principal symbol} matrix is defined as $P(\xi) := \tensor{A}{^i^I_J} \xi_i$.
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where both $A$ and $S$ can depend on $u^I$ but not their derivatives. For any unit covector $\xi_i$, the \emph{Principal symbol} matrix is defined as $\tensor{P}{^I_J}(\xi) := \tensor{A}{^i^I_J} \xi_i$.
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The system is said to be:
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\begin{itemize}
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\item \emph{weekly hyperbolic} if for any $\xi_i$, $P(\xi)$ has only real eigenvalues.
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\item \emph{strongly hyperbolic} if for any $\xi_i$, $P(\xi)$ has only real eigenvalues and a complete set of eigenvectors, i.e., it is diagonalizable.
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\item \emph{weekly hyperbolic} if for any $\xi_i$, $\tensor{P}{^I_J}(\xi)$ has only real eigenvalues.
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\item \emph{strongly hyperbolic} if for any $\xi_i$, $\tensor{P}{^I_J}(\xi)$ has only real eigenvalues and a complete set of eigenvectors, i.e., it is diagonalizable.
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\item \emph{symmetrically hyperbolic} if there exists a positive definite symmetrizer matrix $H_{IJ}$ such that $H_{IK} \tensor{A}{^i^K_J}$ is symmetric for each $i$.
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\end{itemize}
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\end{frame}
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