Align environment

Question

I'd like to ask how in align environment to number only one of the equations.

\begin{aligned}
\mathsf{E}[W^{i}_{t}-W^{i}_{s}]&=0,\\
\mathsf{E}\big[[W^{i}_{t}-W^{i}_{s}][W^{j}_{t}-W^{j}_{s}]\big]&=q_{ij}(t-s),\quad
1\le i,j\le n,\, 0\le s<t.
\end{aligned}

One more question. (The first one has already been sorted out). How can I break page in the align environment?

Answer 1

The aligned environment is a subsidiary math environment and thus can be used within another math environment. For example:

\begin{equation}
\begin{aligned}
\mathsf{E}[W^{i}_{t}-W^{i}_{s}]&=0,\\
\mathsf{E}\big[[W^{i}_{t}-W^{i}_{s}][W^{j}_{t}-W^{j}_{s}]\big]&=q_{ij}(t-s),\quad
1\le i,j\le n,\, 0\le s<t.
\end{aligned}
\end{equation}

This automatically numbers the entire aligned group. But if you are just using it within $ $ or \[ \] there is no numbering. Hence, to get the numbering right, rather use the the align environment as follows:

\documentclass[letterpaper]{article}
\usepackage{amsmath,amssymb}
\begin{document}

\begin{align}
\mathsf{E}[W^{i}_{t}-W^{i}_{s}]&=0,\\
\mathsf{E}\big[[W^{i}_{t}-W^{i}_{s}][W^{j}_{t}-W^{j}_{s}]\big]&=q_{ij}(t-s),\quad
1\le i,j\le n,\, 0\le s<t. \notag \\
\mathsf{E}[W^{i}_{t}-W^{i}_{s}]&=0,
\end{align}

\end{document}

Notice the employment of the \notag command to remove the numbering on desired lines.

This yields:

To work with the page breaks for the align environment, one can use \allowdisplaybreaks. For example:

\documentclass[letterpaper]{article}
\usepackage{amsmath,amssymb,lipsum}
\begin{document}
\lipsum[2-6]
{\allowdisplaybreaks
\begin{align}
\mathsf{E}[W^{i}_{t}-W^{i}_{s}]&=0,\\
\mathsf{E}\big[[W^{i}_{t}-W^{i}_{s}][W^{j}_{t}-W^{j}_{s}]\big]&=q_{ij}(t-s),\quad
1\le i,j\le n,\, 0\le s<t. \notag \\
\mathsf{E}[W^{i}_{t}-W^{i}_{s}]&=0,
\end{align}}
\end{document}

This gives: