# How to make equation display the equation number on the left

When using equation, it shows the equation number on the right of the equation. I want the number (1) to show to the left of the equation:

The code used for generating it is:

$$\mbox{\sc OFFSET}_i=\sum_{j=0}^{i-1} size_j= S-\sum_{j=i}^{M-1} size_j$$
$$=S-size_i\cdot(M-i)-\sum_{j=i}^{M-1} (size_j-size_i)$$
$$=S-size_i\cdot(M-i)-\sum_{j=i}^{M-1}\sum_{\ell=1}^{k}\ell\cdot \mathds {1}_{size_j-size_i=\ell}$$
$$=S-size_i\cdot(M-i)-\sum_{\ell=1}^{k}2^{k-\ell}\cdot\ell$$
$$\label{eq:offset} =S-size_i\cdot(M-i)-(2^{k+1}-k-2)$$


How can I make it show the equation number on the left?

• Please don't use $$...$$ it is not LaTeX syntax. As for the eqn number, see the legno class option for the document class you did not tell us anything about. – daleif Feb 4 '15 at 14:23
• if there is to be only one equation number for such a group, it's more usual to have it either vertically centered, or at the top if it's on the left. is this to be considered a group, or is it really desired to have only the last line numbered? – barbara beeton Feb 4 '15 at 15:00

if only one equation number is wanted for a group of related equations, the equation number is conventionally centered vertically, or placed at the top if the number is at the left.

the following example groups the equations with a single number at the center:

\documentclass[tbtags]{article}
\usepackage[leqno]{amsmath}
\usepackage{dsfont}

\newcommand{\isize}{\mathit{size}}

\begin{document}

$$\begin{split} \text{OFFSET}_i & =\sum_{j=0}^{i-1} \isize_j = S-\sum_{j=i}^{M-1} \isize_j \\ & =S-\isize_i\cdot(M-i)-\sum_{j=i}^{M-1} (\isize_j-\isize_i) \\ & =S-\isize_i\cdot(M-i)-\sum_{j=i}^{M-1}\sum_{\ell=1}^{k}\ell\cdot \mathds 1_{\isize_j-\isize_i=\ell} \\ & =S-\isize_i\cdot(M-i)-\sum_{\ell=1}^{k}2^{k-\ell}\cdot\ell \\ & =S-\isize_i\cdot(M-i)-(2^{k+1}-k-2) \label{eq:offset} \end{split}$$

\end{document}


by adding the tbtags option to amamath:

\usepackage[leqno,tbtags]{amsmath}


the equation number will be placed at the top left:

edit: as suggested by egreg, the string "size" has been made into a word using \mathit for better (and more appropriate) appearance; a command \isize was defined to assist the change.

• It should probably be \mathit{size} throughout. – egreg Feb 4 '15 at 16:51

If you want something like this

you can use an align environment with \notag in all non numbered lines.

amsmath, leqno option will add equation numbers on left side.

Note: I've changed \mathds{1} with just 1. I don't know command \mathds.

\documentclass{article}
\usepackage[leqno]{amsmath}

\begin{document}

\begin{align}
\text{OFFSET}_i & =\sum_{j=0}^{i-1} size_j = S-\sum_{j=i}^{M-1} size_j \notag \\
& =S-size_i\cdot(M-i)-\sum_{j=i}^{M-1} (size_j-size_i) \notag \\
& =S-size_i\cdot(M-i)-\sum_{j=i}^{M-1}\sum_{\ell=1}^{k}\ell\cdot 1_{size_j-size_i=\ell} \notag\\
& =S-size_i\cdot(M-i)-\sum_{\ell=1}^{k}2^{k-\ell}\cdot\ell \notag \\
& =S-size_i\cdot(M-i)-(2^{k+1}-k-2) \label{eq:offset}
\end{align}

\end{document}