[Tex/LaTex] Can’t use \circledast nor ^{º}

Question

sorry if this is a simple question, but I'm new to LaTeX and I'm willing to learn. I want to write this expression:

$r^{º}_{xy}[m]= \lbrace x \circledast \overline{y}^{-}  \rbrace$

but I get an error and a warning. The error states:

! Undefined control sequence. l.90 ...ón $r^{º}_{xy}[m]= \lbrace x \circledast

but if I remove the \circledast the error disappears. The warning has to do with the ^{º}, and it says:

LaTeX Warning: Command \textordmasculine invalid in math mode on input line 90.

Again, if I remove it from the expression then the warning disappears. By the way, I'm using the Palatino font, and I noticed that if I use the ordinary font I don't get these issues in the expression. But I like the Palatino font, so I would like to know how to deal with these problems.

Thanks in advanced

Answer 1

\circledast requires \usepackage{amssymb}.

\documentclass[a4paper]{article}
\usepackage{amssymb}

\begin{document}
$r^{\circ}_{xy}[m]= \lbrace x \circledast \overline{y}^{-}  \rbrace$
\end{document}