* Double dollar
#+begin_src org
  $$CONTENTS$$
#+end_src
becomes
#+begin_src text
  \[CONTENTS\]
#+end_src

#+begin_src org
  $$1+1=2$$
#+end_src

#+begin_src text
  \[1+1=2\]
#+end_src

This gets interpreted by mathjax.
* Single Dollar
#+begin_src org
  PRE$BORDER1 BODY BORDER2$POST
#+end_src
becomes
#+begin_src text
  \(BORDER1 BODY BORDER2\)
#+end_src

#+begin_src org
  foo$a bar b$.
#+end_src
becomes
#+begin_src text
  foo\(a bar b\).
#+end_src

This gets interpreted by mathjax.