I came accross different notations for the multiplication between two quaternions, e.g:
\begin{equation} \mathbf{q}_1 \circ \mathbf{q}_2 \quad \text{or} \quad \mathbf{q}_1 \otimes \mathbf{q}_2 \end{equation}
Which one should be preferred? Is there any standard notation for this operation ? The latter seems to be widely used for the outer product.