Aligning equation number while writing text within equation

Question

I am using text with in aligned environment as

\begin{equation}
\begin{aligned}
\Vert \Vert^2_F&= tr\{()\}\\
 &=\Vert\Vert_F^2\\
\shortintertext{which after differentiation gives}
\mathbf{a}&=[] 
\end{aligned}
\end{equation}

Things are fine, but the equation number is not aligned in center but at the bottom right to last equality $\mathbf{a}&=[]$.

How to align equation number somewhere middle right of whole equation environment.?

Answer 1

It's not common to place an equation number next to a line of text. To achieve this look, you could (a) use an align* environment so that no equation numbers show up on math-mode lines, and (b) set up a helper macro, called \showeq in the example below, that serves to insert a formatted equation number at the end of the line that contains text.

\documentclass{article}
\usepackage{mathtools}
\newcommand\showeq{\hspace{\fill}\refstepcounter{equation}(\theequation)}
\begin{document}

\begin{align*}
\Vert \Vert^2_F&= tr\{()\} \\
               &= \Vert\Vert_F^2 \\
\shortintertext{which after differentiation gives\showeq}
    \mathbf{a} &= []
\end{align*}

\end{document}