MathQuill
Type math here:
LaTeX of what you typed:
<script type="text/javascript" src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
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About
MathQuill is an open source formula editor for the Web maintained by @laughinghan and @stufflebear.
MathQuill was born of a need to communicate math. The creators, @laughinghan and @jneen, used LaTeX for projects together, but struggled to collaborate online once they were apart. They wanted a tool to both write and read math without resorting to ugly flat text or more cumbersome conventions.
After a couple of iterations, MathQuill had become widely useful. Today, MathQuill is used in educational software, math visualizations, collaborative editors, and more.