Infinitesimal proper Euclidean motion
= Infinitesimal proper Euclidean motion
{title2=$V(x)=a\times x+b$}
The derivative at the identity of a smooth curve in $\operatorname{SE}(3)$ is a vector field
$$
V(x)=a\times x+b.
$$
After shifting the origin perpendicular to $a\ne0$, it becomes rotation about the axis parallel to $a$ plus translation along that axis.