Possible Duplicate:
Tex \implies symbol does not work
In this post I cannot get the TeX symbol $\beth$ to work. I checked and the symbol does work on MathOverflow, so it may just be a configuration issue.
I have now made a request about the AMSsymbols extension at http://meta.math.stackexchange.com/questions/1147/please-enable-amssymbols-extension-for-mathjax