I have following code:

    y = \beta_0 + \sum\limits_{i=1}^{m}{\beta_i x_i} + \sum\limits_{i = 1}^{m} \sum\limits_{j \geq 1}^{m}{\beta_{ij} x_i x_j} + \epsilon

    where, & $m$ &= total number of input parameters,\\
     & $x_{i}$ &= $i$th system input,\\
     & $y$ &= system output,\\
     & $\beta_{0}$ &= model constant,\\
     & $\beta_{i}$ &= linear constant,\\
     & $\beta_{ij}$ &= \left \{ \begin{tabular}{l}
                                    interaction constant when $i$\neq$j$,\\
                                    quadratic constant when $i$=$j$,\\
                                 \end{tabular} \\                              
     & $\epsilon$ &= model bias  

With this I get following errors:

    line 553: Missing $ inserted. & $\beta_{ij}$ &= \left
line 554: Missing $ inserted. interaction constant when $i$\neq
line 555: Missing $ inserted. q
line 557: Missing \right. inserted. &
line 557: Missing $ inserted. &
: Unsupported document class (or package) detected,(caption) usage of the caption package is not recommended.
line 501: Underfull \hbox (badness 10000) in paragraph

I am not sure what is causing this exactly. Any idea?

Thank you.

  • 2
    \left\{ is valid only in math mode, and you can't use it without a \right. I'd write just $\{$, maybe with an explicit size declaration. \neq should also be in math mode.
    – campa
    Sep 17, 2015 at 9:19
  • I assume you wanted to have a cases environment with this brace? Also, I would not write $m$= total number of... but rather have a math formula and use \text for "total number...". I think you can achieve your goal with align* instead of the outer tabular.
    – cauchy42
    Sep 17, 2015 at 9:32
  • Which document class are you using? In particular are you in two-column mode?
    – Bernard
    Sep 17, 2015 at 9:41
  • Yes, two column, IEEETRAN.
    – algoProg
    Sep 17, 2015 at 9:44

2 Answers 2


As I said in my comment, \left and \neq can be used only in math mode, and \left always requires a matching \right. Furthermore I wouldn't write $i$=$j$, i.e. I wouldn't exit math mode for the equal sign. I'd type your tabular as

$m$ &= total number of input parameters,\\
$x_{i}$ &= $i$th system input,\\
$y$ &= system output,\\
$\beta_{0}$ &= model constant,\\
$\beta_{i}$ &= linear constant,\\
$\beta_{ij}$ &= $\begin{cases}
                 \text{interaction constant when } i\neq j,\\
                 \text{quadratic constant when } i = j,
$\epsilon$ &= model bias  

I have removed one column, putting the word "where" outside the tabular, and I have used the alignment option [t] to match the first line of the tabular with the word "where" (but that's a matter of taste...). Furthermore, I used the cases environment from amsmath for the conditional definition. The spacing around the = still doesn't look good to me, but I think the issue was to explain the errors...

EDIT: to improve the spacing you could use an align environment

m & =  \text{total number of input parameters,}\\
x_i & =  i\text{th system input,}\\
y & = \text{system output,}\\
\beta_0&=\text{model constant,}\\
  \text{interaction constant when }&i\neq j,\\
  \text{quadratic constant when }&i=j
\epsilon&=\text{model bias}

or if you want the symbols to be left-aligned, an alignat environment (but that's really a matter of taste)

&m &&= \text{total number of input parameters,}\\
&x_{i} &&= i\text{-th system input,}\\
&y &&= \text{system output,}\\
&\beta_{0} &&= \text{model constant,}\\
&\beta_{i} &&= \text{linear constant,}\\
&\beta_{ij} &&= \begin{cases}
                 \text{interaction constant when } i\neq j,\\
                 \text{quadratic constant when } i = j,
&\epsilon &&= \text{model bias} 
  • @campa Thank you. Your overall suggestions to the whole thing with explanations of everything helped me to learn something here. Also your method removed a warning about box full. Thank you again.
    – algoProg
    Sep 17, 2015 at 9:46
  • @campa sorry for taking easy way here, but in align environment how do I put where and first line " m = ...." in the same line?
    – algoProg
    Sep 17, 2015 at 10:05
  • @algoProg Well, if you use align* you could start with \text{where } m&=.... But align* switches to a displayed equation, which is usually centered, and the word "where" would not be aligned to the left. A dirty trick would be to use flalign*, which aligns its content to the left. Then you need a & just before the first line breaking. But I guess the vertical spacing wouldn't be great in that case.
    – campa
    Sep 17, 2015 at 12:57

Use align*:

    m & = & \text{total number of input parameters},\\
    x_i & = & i\text{th system input},\\
    y & =& \text{system output},\\
    \beta_0&=&\text{model constant},\\
      \text{interaction constant when }&i\neq j,\\
      \text{quadratic constant when }&i=j
    \epsilon&=&\text{model bias}

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