# Aligning labels inside cases environment

I have the following code, where I have multiple cases and all need to be labelled. I tried using subnumcases, however I wanted some freedom in labelling (the $k^{th}$ case must be labelled C.k etc), which I could not achieve when using that custom package. I also tried to use two alignment tabs, using alignat, but that does not work inside cases environment.

When I tried to label manually, the labels are not right-aligned, and as such they appear odd. Is it possible to at least right align the manual equation labels? Or is there an elegant way to present these equations?

\documentclass[a4paper]{article}
\usepackage{geometry}
\geometry{left=1in,right=1in,top=1in,bottom=1in}
\usepackage{amsmath, amsfonts, amssymb, bbm, cases}

\begin{document}
\begin{align*}
f(x) =
\begin{cases}
x_1, & \text{if } x\in(\sum_{i=1}^n b_i\log(a_i/b_i)-(\sum_{i=1}^n b_i) \log(a_1/b_1),\infty),\quad \text{(C\textsubscript{1})}\\
x_2, & \text{if } x\in (\sum_{i=2}^n b_i\log(a_i/b_i) -(\sum_{i=2}^n b_i) \log(a_2/b_2), \\ & \sum_{i=1}^n b_i\log(a_i/b_i) -(\sum_{i=1}^n b_i)\log(a_1/b_1)),\quad \text{(C\textsubscript{2})} \\
& \vdots \\
x_k, & \text{if } x\in (\sum_{i=k}^n b_i\log(a_i/b_i) -(\sum_{i=k}^n b_i) \log(a_k/b_k), \\ &\sum_{i=k-1}^n b_i\log(a_i/b_i) -(\sum_{i=k-1}^n b_i)\log(a_{k-1}/b_{k-1})),\quad \text{(C\textsubscript{k})}\\
& \vdots \\
x_n, & \text{if } x\in (0, \sum_{i=n-1}^n b_i\log(a_i/b_i) -(\sum_{i=n-1}^n b_i) \log(a_n/b_n)),\quad \text{(C\textsubscript{n})}.
\end{cases}
\end{align*}
\end{document}

• See the empheq package, I think it is even an example in the manual. Btw: you have an error, there is no A4 class option it us called a4paper – daleif Jun 21 '16 at 16:35
• Off-topic: \geometry{left=1in,right=1in,top=1in,bottom=1in} may be written more succinctly as \geometry{margin=1in}. – Mico Jun 21 '16 at 16:53

The following sets up a specific numcases environment to have the sequential numbering C1, C2, Ck, Cn using \setupnumcases. The setup is restored using \restorenumcases.

The idea is to update \theequation - the macro responsible for setting the equation counter or representation - to set a specific list of possibilities in sequence. This sequence is dictated by \thesaveeqncntr, which is used in place of \theequation.

\documentclass{article}
\usepackage[margin=1in]{geometry}% Just for this example
\usepackage{amsmath,cases}

\newcounter{saveeqncntr}
\renewcommand{\thesaveeqncntr}{\ifcase\value{equation} % 0
\or $C_1$ % 1
\or $C_2$ % 2
\or $C_k$ % 3
\or $C_n$ % 4
\else ? % 5+
\fi}%
\newcommand{\setupnumcases}{%
\setcounter{saveeqncntr}{\value{equation}}% Store equation counter
\setcounter{equation}{0}%
\let\theequation\thesaveeqncntr}
\newcommand{\restorenumcases}{%
\setcounter{equation}{\value{saveeqncntr}}% Restore equation counter
\renewcommand{\theequation}{\arabic{equation}}%
\ignorespaces}

\begin{document}

An equation before
$$f(x) = ax^2 + bx + c.$$

Some text before.%
\setupnumcases
\begin{numcases}{f(x) =}
x_1, & if $x \in (\sum_{i=1}^n b_i\log(a_i/b_i) - (\sum_{i=1}^n b_i) \log(a_1/b_1),\infty)$, \\
x_2, & if $x \in (\sum_{i=2}^n b_i\log(a_i/b_i) - (\sum_{i=2}^n b_i) \log(a_2/b_2)$, \nonumber \\
& $\sum_{i=1}^n b_i\log(a_i/b_i) - (\sum_{i=1}^n b_i)\log(a_1/b_1))$, \\
& $\vdots$ \nonumber \\
x_k, & if $x \in (\sum_{i=k}^n b_i\log(a_i/b_i) - (\sum_{i=k}^n b_i) \log(a_k/b_k)$, \nonumber \\
& $\sum_{i=k-1}^n b_i\log(a_i/b_i) - (\sum_{i=k-1}^n b_i)\log(a_{k-1}/b_{k-1}))$, \\
& $\vdots$ \nonumber \\
x_n, & if $x \in (0, \sum_{i=n-1}^n b_i\log(a_i/b_i) - (\sum_{i=n-1}^n b_i) \log(a_n/b_n))$.
\end{numcases}
\restorenumcases
Some text after. And an equation after
$$f(x) = ax^2 + bx + c.$$

\end{document}


One would have to update \setupnumcases in order to have a different numbering to a differing numcases.

Like this?

Considering comments to question, the code employ empheq package and also environment multlined from package mathtools, which is called by empheq:

\documentclass{article}
\usepackage[a4paper,margin=1in]{geometry}
\usepackage{empheq}

\begin{document}
\begin{empheq}[left={f(x)=}\empheqlbrace]{align}
x_1,    & \text{ if } x\in
\Biggl(\sum_{i=1}^n b_i\log(a_i/b_i)-
\left(\sum_{i=1}^n b_i\right) \log(a_1/b_1),\infty
\Biggr),                 \tag{C\textsubscript{1}}        \\
x_2,    & \begin{multlined}[t][0.6\textwidth]
\text{ if } x\in
\Biggl(\sum_{i=2}^n b_i\log(a_i/b_i) - \sum_{i=2}^n b_i \log(a_2/b_2), \\
\sum_{i=1}^n b_i\log(a_i/b_i) -
\left(\sum_{i=1}^n b_i\right) \log(a_1/b_1)
\Biggr),
\end{multlined}
\tag{C\textsubscript{2}}        \\
& \vdots                \notag                          \\
x_k,    & \begin{multlined}[t][0.6\textwidth]
\text{ if } x\in
\Biggl(\sum_{i=k}^n b_i\log(a_i/b_i)-\sum_{i=k}^n b_i \log(a_k/b_k), \\
\sum_{i=k-1}^n b_i\log(a_i/b_i) -
\left(\sum_{i=k-1}^n b_i\right) \log(a_{k-1}/b_{k-1})
\Biggr),
\end{multlined}
\tag{C\textsubscript{k}}        \\
& \vdots                \notag                          \\
x_n,    & \text{ if }
x\in
\Biggl(0, \sum_{i=n-1}^n b_i\log(a_i/b_i) -
\left(\sum_{i=n-1}^n b_i\right) \log(a_n/b_n)
\Biggr),
\tag{C\textsubscript{n}}
\end{empheq}
\end{document}

• Mathtools does not provide aligned/align (you're using eq numbers), it comes from amsmath. However, if I'm not mistaken, for align I think a modified version of aligned is used, with custom eqn no placement – daleif Jun 21 '16 at 18:09
• huh, now I see mistake, I didn't use aligned, which is from amsmath but multlined, which is from mathtools. Thank you for pointed k me this. – Zarko Jun 21 '16 at 18:47