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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

    \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}
The velocities at time $t$ is the derivative of \eqref{eq:exactSolution}
    \mathbf{\dot{u}}(t) = -k_2 \sin(k_2 t) \mathbf{\dot{u}}(0).

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.

share|improve this question
there's an example in the "additional documentation" file testmath on p.33; pdf file here. the input file and pdf are included in tex live, and the pdf file can be viewed there with texdoc testmath. – barbara beeton Dec 17 '12 at 15:00
up vote 7 down vote accepted

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

  \mathbf{u}(t) &= \cos(k_2 t) \mathbf{u}(0) %

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

share|improve this answer
Worked perfectly first try. – Jeff Dec 22 '12 at 0:53

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