$$
\begin{align}
\sum_{k=1}^{\infty} \frac{1}{k^2} = \frac{\pi^2}{6}
\end{align}
$$

$$ \begin{align} \sum_{k=1}^{\infty} \frac{1}{k^2} = \frac{\pi^2}{6} \end{align} $$

$$
\newcommand{\rot}[1]{\nabla\times #1}
\newcommand{\pdfrac}[2]{\frac{\partial #1}{\partial #2}}
\begin{align}
  \mathbf{D} &= 0 \\\
  \mathbf{B} &= 0 \\\
  \rot{\mathbf{E}} &= - \pdfrac{\mathbf{B}}{t} \\\
  \rot{\mathbf{H}} &= \pdfrac{\mathbf{D}}{t}
\end{align}
$$

$$ \newcommand{\rot}[1]{\nabla\times #1} \newcommand{\pdfrac}[2]{\frac{\partial #1}{\partial #2}} \begin{align} \mathbf{D} &= 0 \\\ \mathbf{B} &= 0 \\\ \rot{\mathbf{E}} &= - \pdfrac{\mathbf{B}}{t} \\\ \rot{\mathbf{H}} &= \pdfrac{\mathbf{D}}{t} \end{align} $$