# Small hollow circle in math, smaller than \circ [duplicate]

I am writing a markdown document and I want to reproduce the following formula:

I'm trying to do it with the following:

$$\underbrace{X_t}_\text{Population at time t} = \underbrace{\alpha \circ X_{t-1}}_\text{Survivors from time t-1} + \underbrace{\epsilon_t}_\text{Immigration}$$


But I get this:

In which the circle operator between $\alpha$ and $X_{t-1}$ is quite bigger (and thinner?) than what it needs to be. How can I solve it?

Edit: Related question: Making a thicker \cdot for dot product (that is thinner than \bullet)

• For some reason, the inline math ($...$) in the question does not render correctly. Why is that happening? – ManuHaq Jan 26 at 19:08
• Thanks, @campa! It does to a great extent (by reducing it to the desired size), but it would be better if also became thicker too, as if the shrunk circle was bolded. – ManuHaq Jan 26 at 19:23

One solution would be to insert a tikz circle, with width and line width that you chose.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}

\newcommand{\circo}{~\raisebox{1pt}{\tikz \draw[line width=0.6pt] circle(1.1pt);}~}
\begin{document}
$$\underbrace{X_t}_\text{Population at time t} = \underbrace{\alpha \circo X_{t-1}}_\text{Survivors from time t-1} + \underbrace{\epsilon_t}_\text{Immigration}$$

\end{document}


• Thanks, this solves it :) – ManuHaq Jan 26 at 20:23