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Thank you! That was the problem.

I am creating a True False table.

Here is a picture of what I have so far:
tablesofar.png
I did that using
p & q \\
in the heading.

For the heading in that 3rd column, I want to write "not p" with the negation symbol.

However,
p & q & \neg p \\
gives me the error:

! Missing $ inserted ...