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


    4 & \hbox{for~odd~$i$} \\
    3 & \hbox{for~even~$i$,}

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



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 A first comment: you shouldn't use eqnarray: it yields bad spacings and is obsolete.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):



  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.

  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*}


enter image description here

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

enter image description here

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:



        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,
        1,&\text{if }i=m,\\
        2,&\text{if }i=1,\\
        5,&\text{if }2 \leq i \leq m-1.

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In plain TeX, it should be done by this:

  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.

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