# How to save space when writing equations with cases?

I am writing in IEEE double column environment. I have some equations with cases. For example, this equation seems to have too much space after the brace and also before and after the commas.

I was using the array environment which seems to cause those large spacing. I still prefer to have some independent control over the alignment of the three parts of the equation, since in some cases for each column, the length may be quite different, and I may choose to center, left, or right align for each column. What would be a good way to make those spacings smaller but still have control over the alignment?

The code I used for the equation is

$$\theta_{pk}=\left\{ \begin{array}{ccc} \omega t+\alpha+\beta & , & n=1,2,3,4,5,6,\ldots\\ -\omega t-\alpha-\beta & , & n=7,8,9,10,11,12,\ldots \end{array}\right.$$

• There exists an environment for that: cases. – marmot Apr 19 at 20:35

as mentioned @marmot in comment, cases from amsmath is right tool for your job. it care for right spacing between building block of your equation:

\documentclass{IEEEtran}
\usepackage{amsmath}

\usepackage{lipsum}  % for dummy text

\begin{document}
\lipsum[1]
$$\theta_{pk} = \begin{cases} \omega t+\alpha+\beta & ,\ n=1,2,3,4,5,6,\ldots\\ -\omega t-\alpha-\beta & ,\ n=7,8,9,10,11,12,\ldots \end{cases}$$
\lipsum[2-4]
\end{document}


addendum: as mentioned @manooooh in his valued comment, AMS (American Mathematical Society) prefer style, where comas follows math expression, i.e:

     \begin{cases}
\omega t+\alpha+\beta,   & n=1,2,3,4,5,6,\cdots,\\
-\omega t-\alpha-\beta,  & n=7,8,9,10,11,12,\cdots.
\end{cases}


which use gives

but some people prefer the following style:

    \begin{cases}
\phantom{-}
\omega t+\alpha+\beta,  & n=1,2,3,4,5,6,\cdots\\
-\omega t-\alpha-\beta,  & n=7,8,9,10,11,12,\cdots
\end{cases}


giving:

however, documentation "amsldoc", as mentioned barbara beeton in her comment, is given the following example, how mathematicians usually use the cases math environment:

    \begin{cases}
\omega t+\alpha+\beta  & n=1,2,3,4,5,6,\cdots,\\
-\omega t-\alpha-\beta  & n=7,8,9,10,11,12,\cdots.
\end{cases}


which gives:

• Is it possible to reduce the space before the commas if I have slightly longer equations to fit in the column? – nanjun Apr 19 at 20:51
• Great answer!! According to AMS, I think that the commas have to start just after the expression e.g. \omega t+\alpha+\beta, and not \omega t+\alpha+\beta&,, and at the end of the line should be a comma e.g. \ldots,. – manooooh Apr 19 at 20:53
• @manooooh -- "According to AMS ... have to" is a bit of a stretch. The traditional location of the commas is indeed at the end of the initial segment of a line, not after the &. That is what is shown in the user guide (texdoc amsldoc). – barbara beeton Apr 20 at 2:55

A variant, with the fleqn environment from nccmath. I improvedged the alignment in the first column of the cases environment, using a phantom — sign in the first row.

You also can save some space – to a certain extent, with the \mathrlap command from mathtools (which loads amsmath). I didn't need it here.

\documentclass{IEEEtran}
\usepackage{amsmath, nccmath}

\usepackage{lipsum} % for dummy text

\begin{document}

\lipsum[1]
\begin{fleqn}
$$\theta_{pk} = \begin{cases} \phantom{-}\omega t+\alpha+\beta, & n=1,2,3,4,5,6,\ldots\\ -\omega t-\alpha-\beta, & n=7,8,9,10,11,12,\ldots \end{cases}$$
\end{fleqn}
\lipsum[2-4]

\end{document}


• What's the purpose of fleqn? I see no reason for it. – egreg Apr 19 at 21:32
• @egreg: Inside this environment, the equations are flushleft, while being aligned, gathered, &c. This can save some space. – Bernard Apr 19 at 21:43