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In each time step we have to solve this problem to get the acceleration $a^{(n)}$ and we will 
In each time step we have to solve this problem to get the acceleration $a^{(n)}$ and we will 
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use the \LinearPDE class of the \linearPDEsPack to do so. The general form of the PDE defined through 
use the \LinearPDE class of the \linearPDEs to do so. The general form of the PDE defined through 
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the \LinearPDE class is discussed in \Sec{SEC LinearPDE}. The form which are relevant here are 
the \LinearPDE class is discussed in \Sec{SEC LinearPDE}. The form which are relevant here are 
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\begin{equation}\label{WAVE dyn 100} 
\begin{equation}\label{WAVE dyn 100} 
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D\hackscore{ij} u\hackscore{j} =  X\hackscore{ij,j}\; . 
D\hackscore{ij} u\hackscore{j} =  X\hackscore{ij,j}\; . 