[Tex/LaTex] Custom equation numbering

equationsnumbering

I want to label an equation

\[ 1 + 2 + \cdots + i = \frac{i(i+1)}{2} \eqname{Inequality(i)} \label{ineq} \]

involving the variable "i" as a function of (i) so the expected output looks like

[LaTeX rendering of the above equation] (2.4)(i)

(which actually means there are "i" such equations). The (2.4) might refer to Equation 4 of Section 2. When I access the equation later, I would like it to appear as

Since Equation (2.4)(n+1) follows from Equation (2.4)(n), use induction…

(EDIT: Improved the wording to clarify the questions in comments. )

Best Answer

Let’s see if I guessed correctly what you wish to get:

This output was generated by the following code:

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{amsmath}

\numberwithin{equation}{section}

\newcommand*{\myTagFormat}[2]{(\ref{#1})($#2$)}



\begin{document}

\section{First}

Just to include a couple of equations:
\begin{align}
      1 &= 1 \label{eq:one} \\
    1+2 &= 3 \label{eq:two}
\end{align}
Until now, we have equations \eqref{eq:one} and~\eqref{eq:two}.

\section{Second}

Still another equation of the same form:
\begin{equation}
    1+2+3 = 6 \label{eq:three}
\end{equation}
Generalizing equations \eqref{eq:one}, \eqref{eq:two}, and~\eqref{eq:three}, we 
claim that
% Little trick:
\refstepcounter{equation}\label{eq:base}% <-- Note!
\begin{equation}
    1+2+\dots+n = \frac{n(n+1)}{2}
    \tag*{\myTagFormat{eq:base}{n}}\label{eq:base-n}
\end{equation}
Now you can either:
\begin{itemize}
    \item
        reference equation~\ref{eq:base-n} directly (note that you must use
        \verb|\ref| instead of \verb|\eqref|, here), or
    \item
        say that equation~\myTagFormat{eq:base}{i+1} follows from
        equation~\myTagFormat{eq:base}{i} by adding $i+1$ on both sides of the
        latter.
\end{itemize}
One more equation to check that we haven't spoiled the numbering:
\begin{equation}
    x=y
\end{equation}

\end{document}

Related Solutions

[Tex/LaTex] Labeled linear program with labeled equations and wide objective function

I've taken some liberties in interpreting what you're trying to accomplish here. For example, I assume that you will manually create the tag names for the LP blocks. Also, I think my solution lacks a certain elegance: in particular, each such block has to be created separately. I don't really like that.

Here's the general idea: create two adjacent boxes centered on the baseline with one containing a dummy equation to create the label for the entire block. Spacing after the boxes is a bit wonky. So, I add ~ after the last flalign environment. (Short coming: you also have to manually adjust the width of the first parbox to get the label to be positioned correctly.)

You seemed to want the first line to somehow align with the LP. I've got two solutions to that. The first puts the first line into a box of zero width. (Short coming: you have to re-enter math mode within this box.) The second puts this first line in its own flalign environment. (Short coming: you have to adjust the vertical spacing between the two environments.)

\documentclass{article}
\usepackage{amsmath,calc}
\newlength{\LPlhbox}
\begin{document}


\settowidth{\LPlhbox}{(P.1)}%
\noindent%
\parbox{\LPlhbox}{\begin{align}
               \tag{P.1}\label{My first LP Block}
               \end{align}}%
\hspace*{\fill}%
\begin{minipage}{\linewidth-2cm}
    \begin{flalign}\notag
     & \makebox[0pt][l]{$\displaystyle{}\max \sum\limits_{v \in V} x_v + \sum\limits_{e \in E} y_e + 10000\gamma$} \\
    \label{this line can be referenced}
     & \text{s.t.} & x_v + x_w   & \geq  0                 && \forall \{v,w\} \in E && \\
     &             & y_3         & \geq 0                  && \forall e\in E        && \\
     &             & (\delta(U)) & \geq \frac{1}{2}(|U|-1) && \forall U \subseteq V
    \end{flalign}~
\end{minipage}

\settowidth{\LPlhbox}{(P.2)}%
\noindent%
\parbox{\LPlhbox}{\begin{align}
               \tag{P.2}\label{My second LP Block}
               \end{align}}%
\hspace*{\fill}%
\begin{minipage}{\linewidth-2cm}
   \begin{flalign}\notag
     \max \sum\limits_{v \in V} x_v + \sum\limits_{e \in E} y_e + 10000\gamma &&
   \end{flalign}
    \vspace{-1.5\baselineskip}
   \begin{flalign}
    & \text{s.t.} & x_v + x_w   & \geq  0                 && \forall \{v,w\} \in E && \\
    &             & y_3         & \geq 0                  && \forall e\in E        && \\
    &             & (\delta(U)) & \geq \frac{1}{2}(|U|-1) && \forall U \subseteq V
   \end{flalign}~
\end{minipage}

Notice that we can reference line~\ref{this line can be referenced}
and both block~\ref{My first LP Block} and block~\ref{My second LP Block}
\end{document}

What would be nice is if there were a commands \lefttag and \righttag to temporarily override document defaults.

You can also easily create a command to do the work of the left-hand box.

\newcommand{\LPblocktag}[2]{\settowidth{\LPlhbox}{(#1)}%
                            \parbox{\LPlhbox}{\begin{align}\tag{#1}#2\end{align}}%
                            \hspace*{\fill}}

I've written the second argument as I have because I don't want to assume you'll be referencing every such block.

So the first block above can be written as

\noindent%
\LPblocktag{P.3}{\label{My third LP Block}}%
\begin{minipage}{\linewidth-2cm}
    \begin{flalign}\notag
     & \makebox[0pt][l]{$\displaystyle{}\max \sum\limits_{v \in V} x_v + \sum\limits_{e \in E} y_e + 10000\gamma$} \\
    \label{this line can be referenced}
     & \text{s.t.} & x_v + x_w   & \geq  0                 && \forall \{v,w\} \in E && \\
     &             & y_3         & \geq 0                  && \forall e\in E        && \\
     &             & (\delta(U)) & \geq \frac{1}{2}(|U|-1) && \forall U \subseteq V
    \end{flalign}~
\end{minipage}

I can reference \ref{My third LP Block}

enter image description here

[Tex/LaTex] Custom automatic equation numbering

You can do this using

\renewcommand{\theequation}{ln.\arabic{equation}}

If you want to switch it back, you can use

\renewcommand{\theequation}{\arabic{equation}}

A complete MWE follows.

\documentclass{article}

\renewcommand{\theequation}{ln.\arabic{equation}}
\begin{document}

\begin{equation}
  y=mx+b
\end{equation}
\end{document}

Best Answer

Related Solutions

[Tex/LaTex] Labeled linear program with labeled equations and wide objective function

[Tex/LaTex] Custom automatic equation numbering

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