Sometimes we have groups of equations that we want lined up nicely:

\begin{eqnarray} \epsilon \frac {{\rm d}^2\psi} {{\rm d}t^2} & = & - \psi \\ q \frac{\partial p} {\partial t} & = & - \nabla . J_{p} - qR \end{eqnarray}

or

\begin{eqnarray} & & \epsilon \frac {{\rm d}^2\psi} {{\rm d}t^2} = - \psi \\ & & q \frac{\partial p} {\partial t} = - \nabla . J_{p} - qR \end{eqnarray}

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