problem with TeX \cases construct

[Stevenparsons]

There is a problem with the \cases command. It doesn't seem to be working properly in MathJax. The cases test from TeXbook pg 175 runs as follows:

$$ |x|=\cases{x,&if $x\ge0$;\cr -x,&otherwise. \cr}$$

MathJax appears to typeset the text in math italic whereas TeX typesets it in an upright font with normal text spacing. See http://screencast.com/t/eX3lcHOJGu for image.

Is anyone else running into this?

[dpvc]

Robert:

Because MathJax doesn't really process text-mode material (only math-mode), its parser doesn't handle the Plain TeX \cases command properly. (To my knowledge, this is the only standard macro that has mixed math/text mode material like this.) You are correct that this is not consistent with actual TeX (one of a handful of differences).

One could use the \text{} macro on the second column entries to make it work, but that is not required in actual TeX. It is one of the things that could be improved, but I haven't looked into what it would take. BTW, jsMath processed this the same way.

Davide

[dpvc]

Well, as a temporary measure, if you add this to your configuration right after the MathJax.Hub.Config() call, it will redefine \cases so that it will work better in most cases.

MathJax.Hub.Register.StartupHook("TeX Jax Ready",function () {
  var TEX = MathJax.InputJax.TeX;
  TEX.Definitions.macros.Cases = TEX.Definitions.macros.cases;
  TEX.Definitions.macros.cases = "myCases";
  TEX.Parse.Augment({
    myCases: function (name) {
      var tex = this.GetArgument(name);
      var tex = '\\begin{cases}' +
                  tex.replace(/\&/g,'\&\\text{').replace(/\\cr/g,'}\\\\') + '}' +
                '\\end{cases}'
      this.string = tex + this.string.slice(this.i); this.i = 0;
    }
  });
});

It also makes the old \cases available as \Cases.

The problem is that it won't work properly if the entries in the \cases include other macros that have arrays in them (e.g., if one of the cases included a matrix). In that case, this "fix" will foul up the equation completely.

The real solution is to write a specialized implementation for \cases, but that is not likely to happen before the new year.

Davide

[dpvc]

I have worked out the details for making \cases typeset the second column in text mode automatically. The cases branch of my fork of MathJax includes the patch.

[fred-wang]

I've prepared a reftest for this issue and check that the branch "cases" fixes it.

==> In testsuite

[fred-wang]

LaTeXToMathML/cases-1.html

[fred-wang]

http://devel.mathjax.org/testing/testsuite/LaTeXToMathML/cases-1.html
http://devel.mathjax.org/testing/testsuite/LaTeXToMathML/cases-1-ref.html