Align after \therefore

Question

If I place my alignment ampersand after a therefore symbol (from amssymb), the proper spacing between the therefore and the rest of the equation is lost. Is there a way to prevent this from happening?

Ex:

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

\begin{document}

\begin{align*}
\text{Characteristic equation: } &r^2 + \lambda^2 c^2 = 0\\
\therefore &r = \pm\lambda ci
\end{align*}

\end{document}

(as compared to placing the ampersand before the therefore symbol)

Answer 1

Insert a {} between \therefore and the & alignment symbol. This restores the proper space of a binary relation.

\begin{align*}
\text{Characteristic equation: } &r^2 + \lambda^2 c^2 = 0\\
\therefore{}&r = \pm\lambda ci
\end{align*}   

produces

instead of

This approach is useful whenever we use symbols defined with \mathrel or \mathbin followed by a & alignment symbol. In this case \therefore is defined in amssymb.sty with \mathrel.