AUCTex: Math mode started with ‘\[’ cannot be closed with dollar in cases*

Question

I have the following code:

\[
  \begin{cases*}
    1& if 
  \end{cases*}
...

after the if, I want to type a dollar sign but get the error message "Math mode started with '\[' cannot be closed with dollar". I am unsure how to fix this.

EDIT: Clarifications

• I want to type a dollar sign to enter into math mode. Note that the cases* environment expects text after the ampersand.
• The error message mentioned is an AUCTeX error (shows up in the *Messages* buffer)---it doesn't allow me to type the dollar sign character into the Emacs buffer.

Answer 1

This behaviour comes from the TeX-insert-dollar function which is described as

(defun TeX-insert-dollar (&optional arg)
  "Insert dollar sign.

If current math mode was not entered with a dollar, refuse to
insert one.  Show matching dollar sign if this dollar sign ends
the TeX math mode and `blink-matching-paren' is non-nil.

When outside math mode, the behavior is controlled by the variable
`TeX-electric-math'.

With raw \\[universal-argument] prefix, insert exactly one dollar
sign.  With optional ARG, insert that many dollar signs."

Looking at the implementation there does not seem to be a possibility to only disable this specific behaviour of refusing to insert $. You basically have two options:

1. Follow the documentation shown above and use the universal argument, i.e. press C-u $ to insert a dollar sign.

2. Remove the $ key from the mode-dependent mapping, i.e.

   (define-key TeX-mode-map "$" nil)
   

   This will map $ back to self-insert-command (or whatever is in your global map) but this way you will also lose the other functionality brought by TeX-insert-dollar such as TeX-electric-math.