Problem with aligned environment in latex?
aligned
takes an optional argument and so it is mistakening [X]
for that option. The way to overcome is to place something innocuous after the {aligned}
, but before the [X]
. Here I place an empty group {}
.
Note that the option, when employed, is a vertical-alignment positioning specification, such as [t]
or [b]
.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}{}
[X] &= 1\\
[Y] &= 2,
\end{aligned}
\end{equation}
\end{document}
Another solution is to load the mathtools package. The mathtools
package is a (very significant!) superset of the amsmath
package which also fixes a couple of de facto bugs -- such as mis-interpreting [X]
as the optional argument of \begin{aligned}
-- of the amsmath
package. For more about this bug, see section 2.1 of the user guide of the mathtools
package.
To wit,
\documentclass[leqno]{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\begin{aligned}
[X] &= 1\\
[Y] &= 2,
\end{aligned}
\end{equation}
\end{document}
works exactly as expected:
A nice aspect of this solution is that you needn't remember to insert {}
after \begin{aligned}
if the next line begins with an object -- such as [X]
-- that's encased in square brackets.