[OS X TeX] Missing $ inserted in a "variations" environment

[Iraj Kalantari]

Setting aside Jonathan's wonderful analysis, if in the minimal example  
provided
one replaces the first '\mI' with '$\mI$'
in the line:
\m{f(x)} & \bg \mI &  \c \h{\pI} & \bb& \mI & \c & \h{-1} & \d \mI & \bb
the problem goes away.

Iraj.




On Mar 15, 2009, at 12:39 PM, Jonathan Kew wrote:

> On 15 Mar 2009, at 14:09, <ewan.Delanoy at math.unicaen.fr> <ewan.Delanoy at math.unicaen.fr 
> > wrote:
>
>>                  Hello all,
>>
>>
>>  I've just encountered the following problem with TeXShop :
>
> You're having a problem with LaTeX, not with TeXShop. It would be  
> useful to learn something about the distinction.
>
>>
>> 1) Compilation stops with the error message "Missing $ inserted".
>> 2) If I insist and press "Enter" twice in the console, it eventually
>> compiles completely
>> and produces a fine output.
>
> Not quite. Look at the first minus-infinity at the bottom; it'll  
> have a hyphen instead of a minus sign.
>
>>
>> 3) By trial and error, I see that what causes this behaviour is a
>> "variations" array
>> (from the variations package) in the middle of my text.
>> 3) The line number displayed in the error message, l.259, is not very
>> illuminating
>> because the source text is as follows
>
> This is presumably because none of the content of the {variations}  
> environment actually gets processed until the end reached.
>
>>
>> \noindent \makebox[\textwidth][c]{
>> \begin{variations}
>>  x  & \mI  &   & \alpha& & & 1 & & \beta&  & & 2 & & \pI & \\
>>  \filet
>> f'(x) & \bg & + & \bb&  & + & \z & -& \bb & & - &\z& + & &\bd \\
>> \filet
>> \m{f(x)} & \bg \mI &  \c \h{\pI} & \bb& \mI & \c & \h{-1} & \d \mI  
>> & \bb
>> & \h{\pI}& \d &\frac{3}{2}& \c
>> & \h{\pI} & \bd \\
>> \end{variations}
>> }<- Line 259 is here
>>
>> Can anyone help me on this ?
>
> One way to locate the problem more precisely, if you don't notice  
> the incorrect minus sign at first, is to type "i XXXX" in response  
> to the error message instead of just hitting Enter. You'll see the  
> inserted XXXX in your output and this may help you spot where things  
> are going wrong.
>
> It seems that {variations} is automatically putting each "cell" of  
> the table it creates into math mode, probably using an \halign  
> template. But the \bg command, besides creating an extra rule on the  
> left, also cancels the alignment template for the current entry  
> (\show\bg indicates that its expansion  begins with \omit). So where  
> you have "....& \bg \mI &...." the automatic math mode does not  
> apply, and hence you get an error when you try to use the infinity  
> symbol.
>
> You can remove the error by adding $...$ around \mI in this case,  
> but (a) it seems ugly to have to do this, when other cells are  
> automatically in math mode; and (b) this results in extra space  
> between the symbol and the following arrow. So it's not really very  
> satisfactory.
>
> It looks like what you should really do is split off the border  
> you're creating with \bg into a separate column of the table, as the  
> {variations} environment doesn't know how to combine this nicely  
> with math-mode content. Thus:
>
> 	\noindent \makebox[\textwidth][c]{
> 	\begin{variations}
> 	x & & \mI & & \alpha& & & 1 & & \beta& & & 2 & & \pI & \\
> 	\filet
> 	f'(x) & \bg & & + & \bb& & + & \z & -& \bb & & - &\z& + & &\bd \\
> 	\filet
> 	\m{f(x)} & \bg & \mI & \c \h{\pI} & \bb& \mI & \c & \h{-1} & \d \mI  
> & \bb
> 	& \h{\pI}& \d &\frac{3}{2}& \c
> 	& \h{\pI} & \bd \\
> 	\end{variations}
> 	}
>
> HTH,  JK
>
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