I am using textlive2014 (and I had the same error in 2013 and before though). Only now I decided to do anything about it.
Whenever I type a formula containing
$\o$ I get the warning
Command \o is invalid in mathmode. But it displays fine, specifically, it gives the empty set symbol but with a small
o instead of
What causes this warning? How can it be gotten rid of? Is there now a different command for the empty set with lowercase o instead of
\o and the warning means I am using an outdated command?
$\O$ never gives me any problems and no problems in the output either way, only a warning in the