# How to align all equalities?

I am a newbie here and I got stuck in the following situation. Let me type the portion first

\documentclass{article}

\begin{document}
\begin{eqnarray*}
f_2(u_iu_{i+1})&=&\left\{
\begin{array}{ll}
4 & \hbox{for~odd~$i$} \\
3 & \hbox{for~even~$i$,}
\end{array}
\right.\\[.2cm]
f_2(v_mv_1)&=&5,\\
f_2(u_mu_1)&=&5,\\

for $2\leq i\leq m-1,$

f_2(v_iu_i)&=&5,\\
f_2(v_mu_m)&=&1,\\
f_2(v_1u_1)&=&2.
\end{eqnarray*}

\end{document}


My problem is: I want the sentence "for $2\leq i\leq m-1,$" at the extreme left while all other equality signs shown above should be aligned. But I falied to maintained that. Please someone help me on it.

P.S. Please feel free to edit the above if necessary.

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Welcome to TeX SX! A first comment: you shouldn't use eqnarray: it yields bad spacings and is obsolete. Use one of the amsmath environments instead (align,aligned, &c.) – Bernard Mar 24 at 10:00

I propose two solutions, based on align*, and the cases* environment from mathtools (it is like the standard cases, except the second column is typeset in text mode):

\documentclass{article}
\usepackage{mathtools}
\usepackage[showframe]{geometry}

\begin{document}

\begin{align*}
f_2(u_iu_{i+1}) & =\begin{cases*}
4 & for odd $i$ \\
3 & for even $i$, \end{cases*} \\
f_2(v_mv_1) & = 5, \\
\text{for $2\leq i\leq m-1$,}
\quad f_2(v_iu_i) & = 5, \\
f_2(v_mu_m) & = 1, \\
f_2(v_1u_1) & = 2.
\end{align*}

\begin{align*}
f_2(u_iu_{i+1})&=\begin{cases*}
4 & for odd $i$\\
3 & for even $i$,
\end{cases*} \\
f_2(v_mv_1)&= 5,\\
f_2(v_iu_i) & = \begin{cases*}5 & for $2\leq i\leq m-1$, \\
1 & for $i = m$,\\
2 & for $i = 1$. \end{cases*}
\end{align*}

\end{document}


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You can combine several of these branches so I'd go for something like this:

Rather than eqnarray I recommend using the align* from the amsmath package. There is also a cases environment for typesetting piece-wise functions and a \text{...} command for writing text inside equations. Putting these together, here is the code I used:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
f_2(u_iu_j)&=\begin{cases}
3,&\text{if $i$ is odd and }j=i+1,\\
4,&\text{if $i$ is even and }j=i+1,\\
5,&\text{if $i=m$ and }j=1,
\end{cases}\\
f_2(v_iu_i)&=\begin{cases}
1,&\text{if }i=m,\\
2,&\text{if }i=1,\\
5,&\text{if }2 \leq i \leq m-1.
\end{cases}
\end{align*}

\end{document}

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In plain TeX, it should be done by this:

\eqalignno{ f_2(u_i u_{i+1}) &= \cases{4 & for odd i, \cr 3 & for even i,} \cr f_2(v_m v_1) &= 5, \cr f_2(u_m u_1) &= 5, \cr \noalign{\hbox{for 2 \leq i \leq m-1,}} f_2(v_i u_i) &= 5, \cr f_2(v_m u_m) &= 1, \cr f_2(v_1 u_1) &= 2. }

\bye

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