Using \tag in unnumbered versions

Question

According to the amsmath documentation (section 3.1, page 7)

\tag and \tag* can also be used within the unnumbered versions of all the amsmath alignment structures

It provides no examples (surprise!). But my code

\documentclass[12pt,fleqn,reqno]{article}
\usepackage{amsfonts,graphics,epsfig,cite}
\usepackage{amsmath,graphicx,amssymb,amsthm}
\begin{document}
\begin{align*}
    \mathbf{u}(t) &= 2\sum_{n=1}^N A_n \cos(k_n t) \mathbf{v}_n \\
        &= 2\left(A_2 \cos(k_2 t)\mathbf{v}_2 + A_N \cos(k_N t)\mathbf{v}_N\right) \\
        &= 2\left(A_2 \cos(k_2 t)\mathbf{v}_2 + A_2^* \cos(k_2 t)\mathbf{v}_2^*\right) \\
        &= 2 \cos(k_2 t) \left(A_2 \mathbf{v}_2 + A_2^* \mathbf{v}_2^*\right) \\
        &= 2 \cos(k_2 t) \big(A_2 \mathbf{v}_2 + (A_2 \mathbf{v}_2)^*\big) \\
        &= 2 \cos(k_2 t) 2 \Re(A_2 \mathbf{v}_2) \\
        &= 4 \cos(k_2 t) \Re(A_2 \mathbf{v}_2) \\
... \\
    \mathbf{u}(t) &= \cos(k_2 t) \mathbf{u}(0) \tag{eq:exactSolution}
.\end{align*}
The velocities at time $t$ is the derivative of \eqref{eq:exactSolution}
\begin{equation*}
    \mathbf{\dot{u}}(t) = -k_2 \sin(k_2 t) \mathbf{\dot{u}}(0).
\label{eq:}
\end{equation*}
\end{document}

generates the warning

Reference 'eq:exactSolution' on page 1 undefined on input line 16

even though "eq:exactSolution" is defined 2 lines above the warning. Please tell me how to fix this.

Answer 1

\tag{<stuff>} prints <stuff> as the actual tag, not the label. So you need to use

\begin{align*}
  %...
  \mathbf{u}(t) &= \cos(k_2 t) \mathbf{u}(0) %
    \stepcounter{equation}\tag{\theequation}\label{eq:exactSolution}
  %...
\end{align*}
...\eqref{eq:exactSolution}...

In the above example, I've stepped the equation counter and used it in the \tag, for consistency. However, you could have used anything.