Are the \[ and \begin{equation} used interchangeably?

Question

When I use double dollar signs $$ or \[ and \] to begin and end math mode I have no problem compiling the file. When I substitute them with \begin{equation} and \end{equation} correspondingly I get an error.

For example:

\[
\label{eq:stress_strain_eq}
\left\{\begin{array}{c}
\sigma_x \\
\sigma_y \\
\tau_{xy}
\end{array}\right\}
=
\begin{bmatrix}
\overline{Q}_{11}   & \overline{Q}_{12}  & \overline{Q}_{16}    \\[0.3em]
\overline{Q}_{12}   & \overline{Q}_{22}  & \overline{Q}_{26}    \\[0.3em]
\overline{Q}_{16}   & \overline{Q}_{26}  & \overline{Q}_{66}
\end{bmatrix}
\cdot
\left\{\begin{array}{c}
\epsilon_x \\
\epsilon_y \\
\gamma_{xy}
\end{array}\right\}
\]

produces the right result but I cannot reference it. I thought that the three options were interchangeable.

Answer 1

The environments

\begin{equation}
 ...
\end{equation}

and

\[ ... \]

are not interchangeable, as the first will produce an equation number, while the latter will not. A \label{...} command in the latter will not produce an error, but it has no effect since there is no counter it may refer to.

The equivalent to the latter would be the starred version of the equation environment, which is provided by the amsmath package:

\begin{equation*}
 ...
\end{equation*}