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If you try to show this formula in comments, it fails.

$$M:\left[\begin{array}{cc}x&y\end{array}\right]\mapsto\left[\begin{array}{ccc}x&y&1\end{array}\right]M=\left[\begin{array}{ccc}u&v&w\end{array}\right]\mapsto\left[\begin{array}{cc}\frac{u}{w}&\frac{v}{w}\end{array}\right]$$

Source here.

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    $\begingroup$ Example $$M:\left[\begin{array}{cc}x&y\end{array}\right]\mapsto\left[\begin{array}{ccc}x&y&1\end{array}\right]M=\left[\begin{array}{ccc}u&v&w\end{array}\right]\mapsto\left[\begin{array}{cc}\frac{u}{w}&\frac{v}{w}\end{array}\right]$$ $\endgroup$ – hhh Sep 14 '12 at 1:22
  • $\begingroup$ See also: meta.math.stackexchange.com/questions/3953/… $\endgroup$ – Martin Sleziak Jun 2 '15 at 19:34
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In comments, as in chat, when a string has more than 80 characters without whitespace, the editor adds whitespace (\unicode{x200C}\unicode{x200B} Zero Width Non-Joiner, Zero Width Space). If you get lucky, the whitespace is added in a place where it does not cause a problem. However, most of the time, it breaks a keyword, control sequence, etc and corrupts the LaTeX.

Edit: I removed the unicode whitespace from its bad location and added ascii whitespace (0x20) in good places. The LaTeX now works in the comment below.

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    $\begingroup$ Let's see if this works with spaces forced in good places $$M:\left[\begin{array}{cc}x&y\end{array}\right] \mapsto\left[\begin{array}{ccc}x&y&1\end{array}\right] M=\left[ \begin{array}{ccc}u&v&w\end{array}\right] \mapsto\left[\begin{array}{cc}\frac{u}{w} &\frac{v}{w}\end{array}\right]$$ $\endgroup$ – robjohn Sep 14 '12 at 2:34
  • $\begingroup$ Why is the whitespace automatically added? $\endgroup$ – user26486 Feb 2 '15 at 1:14
  • $\begingroup$ @user314: the zero-width spacing characters are there to tell the page layout software that it is legal to break a line at that point. Excessively long lines without any breaks would break the page layout software; lines would extend into areas reserved for other parts of the page. This is what happened in the comment to the question. The whitespace is added, it breaks the LaTeX, and the MathJax renderer does not reinsert the special characters into the erroneous output so the line is too long. $\endgroup$ – robjohn Feb 2 '15 at 2:12
  • $\begingroup$ @JyrkiLahtonen: I am simply explaining why the whitespace is added, not whether it is the right thing to do. BTW, how would a browser know where to break the word you cite? Should browsers be equipped with dictionaries of all languages? Compound words make even this seem too meager. $\endgroup$ – robjohn Mar 6 '16 at 17:24

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