I have written in my work $X^{+},\,\rm{e}^{X},\,|X|^{p}$. Why is the second and third term not in italics?
[Tex/LaTex] How to write maths symbols
math-modesymbols
Related Solutions
There is a package to do this, appropriately called slashed.
By default LaTeX is in text mode. In LaTeX all mathematics needs to be inside some maths mode contruction and many symbols, including \circ, are defined to only work in maths mode.
As the other anwsers show the standard way to cope with this is to place \circ between dollar signs: $\circ$. An alternative, that may become the default in future, is to use \( \circ \). Single dollar $ toggles between math and text mode; \( switches to maths mode from text mode and issues an error otherwise. See Are \( and \) preferable to dollar signs for math mode? for some discussion.
(Another choice that could be useful is \ensuremath{\circ}, but this is more a programming construction.)
On the other hand \circ may not be producing the symbol you wish. A simple dot that is vertically centered is produced by \cdot:
\documentclass{article}
\begin{document}
Some text \( \circ \) and more and a centered dot \( \cdot \) and more.
\end{document}
See How to look up a symbol or identify a math symbol or character? for discussion on finding symbols. Note that you will find that some symbols have variants starting with \text... that can be legitimately used in text mode.
Best Answer
Because you used
\rminstead of\mathrm.\rmis not a macro which accepts an argument, as you apparently thought. The macro which does that is\textrm{text}for text mode and\mathrm{text}for math mode. Instead, macro\rmchanges the current font to roman, from the point in which the macro appears until the end of the current scope (which is the end of the math mode in your case, but more generally the end of the group in which\rmappears).Macros
\it,\bfand\tt, work in the same line than\rm, and they have also a version which accepts one parameter, which are\textit{}(but more frequently\emph{}),\textbf{}and\textt{}.Those two-letter macros which change fonts and styles are deprecated and should not be used. They are maintained for backwards compatibility.