[Tex/LaTex] Numbering specific lines in align environment

alignnumbering

I want to number specific lines of an equation in the align environment. I've seen how to use gather instead of align here, but putting \usepackage[fleqn]{amsmath} in the preamble messes with the other equations in the document.

\documentclass[11pt,a4paper,openright]{book}
\usepackage[latin1]{inputenc}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage[left=1.00in, right=1.00in, top=1.00in, bottom=1.20in]{geometry}
\usepackage[hidelinks]{hyperref}

\begin{document}

For any test function $\phi\in\mathcal{D}$,
\begin{align}
\langle f'(t),\phi(t) \rangle &= \int_{-\infty}^{\infty} f'(t)\phi(t) \mathrm{d}t \\ 
&= [f(t)\phi(t)]_{-\infty}^\infty - \int_{-\infty}^{\infty} f(t)\phi'(t) \mathrm{d}t \\ \nonumber
&= -\langle f(t),\phi'(t) \rangle
\end{align}

\end{document}

I want to number the first and third lines, not the first and second.

Best Answer

In each line of your align environment, \\ ends the equation. So, any actions you want to perform on that particular line need to go before the ending \\.

enter image description here

\documentclass{article}
\usepackage{amsmath}

\begin{document}

For any test function $\phi\in\mathcal{D}$,
\begin{align}
  \langle f'(t),\phi(t) \rangle &= \int_{-\infty}^{\infty} f'(t)\phi(t) \mathrm{d}t \\ 
    &= [f(t)\phi(t)]_{-\infty}^\infty 
         - \int_{-\infty}^{\infty} f(t)\phi'(t) \mathrm{d}t \nonumber \\ %%%%%%%%
    &= -\langle f(t),\phi'(t) \rangle
\end{align}

\end{document}

Related Solutions

[Tex/LaTex] Numbering color in align environment

I checked the AMS LaTeX documentation amsmath.pdf to see how labels are treated, and I found a way to colour the equation number next to the equation red, while the colour of any reference is unchanged. All other parts of the align environment have to be made red separately. One thing to note: If you use \color{red} within the environment align, its effect only reaches until the next & or \\. Apparently, all cells in the environment are within different groups.

\makeatletter
\definecolor{johnnywalker}{rgb}{1,0,0}
\newcommand{\redlabel}{%
  \refstepcounter{equation}%
  \global\tag@true%
  \nonumber%
  \gdef\df@tag{\maketag@@@{{\color{johnnywalker}(\theequation)}}\def\@currentlabel{\theequation}}}
\makeatother

Then you can use this as follows:

\begin{align}
  a &  = b + c
  \\
  \redlabel
  &  = e + f
\end{align}

Now, \ref and \eqref should behave as expected.

Explanation

The commands \tag and \label do not create the tag and label immediately. Instead, everything is deferred until all of the line has been read. Only then is the tag typeset and the label created.

The code above does the following:

Step the counter manually (since we cannot rely on the automatic tag generation)
Tell AMSTeX that the tag has been set manually.
Tell AMSTeX to not generate an equation number automatically
Initialize the command \df@tag with content that generates the tag and the label
1. and 3. I copied from the definition of \tag@in@align (the version of \tag that is active in align environments). 4. comes from the definition of \make@df@tag@@@ (the command that is responsible for saving the information about the tag and the label).

[Tex/LaTex] Numbering align-environments

Either use \numberwithin{equation}{section} from amsmath or mathtools or the chngcntr package (if more than this is needed, e.g. \counterwithout or the starred versions \counterwithin* etc.)

\documentclass{article}

\usepackage{chngcntr}

\usepackage{mathtools}
\counterwithin{equation}{section}


\begin{document}
\section{A section}
\begin{align}
  E &= mc^{2} \\
  E^2 &= p^{2}c^{2} + m^{2}c^{4}
\end{align}


\section{Another section}

\begin{align}
  E &= mc^{2} \\
  E^2 &= p^{2}c^{2} + m^{2}c^{4}
\end{align}

\counterwithout{equation}{section} % Remove the resetting by `section`

\section{Yet another section}

\begin{align}
  E &= mc^{2} \\
  E^2 &= p^{2}c^{2} + m^{2}c^{4}
\end{align}

\end{document}

Best Answer

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[Tex/LaTex] Numbering color in align environment

Explanation

[Tex/LaTex] Numbering align-environments

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