Separate subequations over two lines each

Question

My working example is:

\documentclass[a4paper, 12pt]{scrreprt}

\usepackage{amsmath}
\usepackage{braket}
\usepackage{mleftright}

\begin{document}

\begin{subequations}
\label{ComputationInnProd}
    \begin{align}
        \label{ComputationInnProdDiscr}
        \langle \underbrace{ \mleft( \sum_{\alpha'} \psi_{\alpha'}^* \bra{\alpha'} \mright) }_{\bra{\psi}} \vert \underbrace{ \mleft( \sum_{\alpha''} \varphi_{\alpha''} \ket{\alpha''} \mright) }_{ \ket{\phi} } \rangle = \sum_{\alpha', \alpha''} \psi_{\alpha'}^* \varphi_{\alpha''} \braket{\alpha' \vert \alpha''} = \\
        \sum_{\alpha',\alpha''} \psi_{\alpha'}^* \varphi_{\alpha''} \delta_{\alpha' \alpha''} = \sum_{\alpha'} \psi_{\alpha'}^* \varphi_{\alpha'},  \\
        \label{ComputationInnProdCont}
        \langle \underbrace{ \mleft( \int {d\alpha'} \psi^* \mleft( \alpha' \mright) \bra{\alpha'} \mright) }_{\bra{\psi}} \vert \underbrace{ \mleft( \int {d\alpha''} \varphi \mleft( \alpha'' \mright) \ket{\alpha''} \mright) }_{ \ket{\phi} } \rangle = \int {d\alpha'} \int {d\alpha''} \psi^* \mleft( \alpha' \mright) \varphi \mleft( \alpha'' \mright) \braket{\alpha' \vert \alpha''} = \\
        \int {d\alpha'} \int {d\alpha''} \psi^* \mleft( \alpha' \mright) \varphi \mleft( \alpha'' \mright) \delta \mleft( \alpha' - \alpha'' \mright) = \int {d\alpha'} \psi^* \mleft( \alpha' \mright) \varphi \mleft( \alpha' \mright),
    \end{align}
\end{subequations}

\end{document}

I want the first two equations to belong to subequation 0.1a and have the ComputationInnProdDiscr label and the latter two to be grouped as subequation 0.1b and have the label ComputationInnProdCont for future reference.

This code produces the following output:

I've marked each of the equality signs that I want to be aligned with each other in red and green, respectively. I'm also open to suggestions for what would make this more readable because it does not look particularly pretty.

Use \begin{split} \end{split}. See here https://tex.stackexchange.com/questions/17528/show-equation-number-only-once-in-align-environment — Elad Den, May 20 '20 at 09:57

score 5 · Accepted Answer · answered May 20 '20 at 10:28

In addition to applying some judiciously-chosen \notag instructions, you should also reconsider the objective of displaying the subequations across just 2 lines. I think things would be a lot clearer, and less cramped, if they were typeset across 3 lines each.

Note the use of the \smash[b]{...} directives. They serve to reduce (actually, eliminate) the extra vertical whitespace that would otherwise get inserted after lines 1 and 4.

Finally, don't overuse \mleft and \mright. (In the following solution, they aren't used at all.) On the other hand, don't neglect to increase the sizes of the angle brackets either.

\documentclass[a4paper, 12pt]{scrreprt}
\usepackage{amsmath,braket}

\begin{document}
\setcounter{chapter}{3} % just for this example
\begin{subequations}
\label{ComputationInnProd}
\begin{align}
\label{ComputationInnProdDiscr}
\smash[b]{\biggl\langle 
\underbrace{ \biggl( \sum_{\alpha'} \psi_{\alpha'}^* \bra{\alpha'} \biggr) }_{\bra{\psi}} \biggm\vert 
\underbrace{ \biggl( \sum_{\alpha''} \varphi_{\alpha''} \ket{\alpha''} \biggr) }_{ \ket{\phi} } \biggr\rangle} 
&= \sum_{\alpha', \alpha''} \psi_{\alpha'}^* \varphi^{}_{\alpha''} \braket{\alpha' \vert \alpha''}  \notag\\
&=\sum_{\alpha',\alpha''} \psi_{\alpha'}^* \varphi^{}_{\alpha''} \delta_{\alpha' \alpha''} \notag\\
&= \sum_{\alpha'} \psi_{\alpha'}^* \varphi^{}_{\alpha'}\,\text{;}  \\
\label{ComputationInnProdCont}
\smash[b]{\biggl\langle 
\underbrace{ \biggl( \int\!d\alpha' \psi^* ( \alpha') \bra{\alpha'} \biggr) }_{\bra{\psi}} \biggm\vert 
\underbrace{ \biggl( \int\!d\alpha'' \varphi ( \alpha'') \ket{\alpha''} \biggr) }_{ \ket{\phi} } \biggr\rangle}  
&= \int\!d\alpha' \int\!d\alpha''\, \psi^* ( \alpha') \varphi ( \alpha'') \braket{\alpha' \vert \alpha''}  \notag \\
&=\int\!d\alpha' \int\!d\alpha''\, \psi^* ( \alpha') \varphi ( \alpha'') \delta ( \alpha' - \alpha'') \notag \\
&= \int\!d\alpha'\, \psi^* ( \alpha') \varphi ( \alpha')\,.
\end{align}
\end{subequations}

\end{document}

egreg · Answer 2 · 2020-05-21T07:29:00.027

Here's my proposal.

Remove all \mleft and \mright.
Use manual sizing for \langle and \rangle.
Divide the alignment into two nested aligned environments.
Smash the bottom when the underbrace is involved.

The code:

\documentclass[a4paper, 12pt]{scrreprt}

\usepackage{amsmath}
\usepackage{braket}

\begin{document}

\begin{subequations}
\label{ComputationInnProd}
\begin{align}
\label{ComputationInnProdDiscr}
&\begin{aligned}[b]
\smash[b]{
\biggl\langle
  \underbrace{
     \sum_{\alpha'} \psi_{\alpha'}^* \bra{\alpha'} 
  }_{\bra{\psi}}
\biggm|
  \underbrace{
     \sum_{\alpha''} \varphi_{\alpha''} \ket{\alpha''} 
  }_{\ket{\phi}}
\biggr\rangle
}% end smash
&= \sum_{\alpha', \alpha''} \psi_{\alpha'}^* \varphi_{\alpha''} \braket{\alpha' \vert \alpha''}
\\
&= \sum_{\alpha',\alpha''} \psi_{\alpha'}^* \varphi_{\alpha''} \delta_{\alpha' \alpha''}
 = \sum_{\alpha'} \psi_{\alpha'}^* \varphi_{\alpha'}, 
\end{aligned}
\\[2ex]
\label{ComputationInnProdCont}
&\begin{aligned}[b]
\smash[b]{
\biggl\langle
  \underbrace{
     \int d\alpha'\, \psi^* (\alpha') \bra{\alpha'} 
  }_{\bra{\psi}}
\biggm|
  \underbrace{
     \int d\alpha''\, \varphi (\alpha'') \ket{\alpha''} 
  }_{ \ket{\phi} }
\biggr\rangle
}% end smash
&= \int d\alpha' \int d\alpha''\, \psi^* (\alpha') \varphi (\alpha'')
        \braket{\alpha' \vert \alpha''}
\\
&= \int d\alpha' \int d\alpha''\, \psi^* (\alpha') \varphi (\alpha'')
        \delta (\alpha' - \alpha'')
\\
&= \int d\alpha'\, \psi^* (\alpha') \varphi (\alpha'),
\end{aligned}
\end{align}
\end{subequations}

\end{document}

Note the \, to separate the various differentials from the function or a following differential.

don't you define \ub but then never use it? – Markus Gratis May 21 '20 at 05:18 — Markus Gratis, May 21 '20 at 05:18
@MarkusGratis Oh, that remained from a previous attempt. – egreg May 21 '20 at 07:28 — egreg, May 21 '20 at 07:28

score 2 · Answer 3 · edited May 20 '20 at 10:31

Elad Den was right in forwarding me to the linked thread on tex stack exchange. The solution for this specific case is:

\begin{subequations}
\label{ComputationInnProd}
    \begin{align}
        \begin{split}
        \label{ComputationInnProdDiscr}
        \langle \underbrace{ \mleft( \sum_{\alpha'} \psi_{\alpha'}^* \bra{\alpha'} \mright) }_{\bra{\psi}} \vert \underbrace{ \mleft( \sum_{\alpha''} \varphi_{\alpha''} \ket{\alpha''} \mright) }_{ \ket{\phi} } \rangle & = \sum_{\alpha', \alpha''} \psi_{\alpha'}^* \varphi_{\alpha''} \braket{\alpha' \vert \alpha''} = \\
        \sum_{\alpha',\alpha''} \psi_{\alpha'}^* \varphi_{\alpha''} \delta_{\alpha' \alpha''} & = \sum_{\alpha'} \psi_{\alpha'}^* \varphi_{\alpha'},
        \end{split}
    \end{align}
    \begin{align}
        \begin{split}
        \label{ComputationInnProdCont}
        \langle \underbrace{ \mleft( \int {d\alpha'} \psi^* \mleft( \alpha' \mright) \bra{\alpha'} \mright) }_{\bra{\psi}} \vert \underbrace{ \mleft( \int {d\alpha''} \varphi \mleft( \alpha'' \mright) \ket{\alpha''} \mright) }_{ \ket{\phi} } \rangle & = \int {d\alpha'} \int {d\alpha''} \psi^* \mleft( \alpha' \mright) \varphi \mleft( \alpha'' \mright) \braket{\alpha' \vert \alpha''} = \\
        \int {d\alpha'} \int {d\alpha''} \psi^* \mleft( \alpha' \mright) \varphi \mleft( \alpha'' \mright) \delta \mleft( \alpha' - \alpha'' \mright) & = \int {d\alpha'} \psi^* \mleft( \alpha' \mright) \varphi \mleft( \alpha' \mright),
        \end{split}
    \end{align}
\end{subequations}

Encapsulating everything in one align environment is ambigous regarding the labels and leaving out the aligns (only split) alltogether produces an error.

Separate subequations over two lines each

3 Answers3