Use S^3 for unit sphere

doc/source/user_guide/rotations.rst

@@ -314,7 +314,7 @@ Read `this paper <https://arxiv.org/pdf/1801.07478.pdf>`_ for details about
 the two conventions and why Hamilton's convention should be used. Section VI A
 gives further useful hints to identify which convention is used.
 
-The unit quaternion space :math:`\mathbb{S}^3` can be used to represent
+The unit quaternion space :math:`S^3` can be used to represent
 orientations with an encoding based on the rotation axis and angle.
 A rotation quaternion is a four-dimensional unit vector (versor)
 :math:`\boldsymbol{\hat{q}}`.

pytransform3d/rotations/_quaternions.py

@@ -158,7 +158,7 @@ def q_prod_vector(q, v):
     We use Hamilton's quaternion multiplication.
 
     To apply the rotation defined by a unit quaternion :math:`\boldsymbol{q}
-    \in \mathbb{S}^3` to a vector :math:`\boldsymbol{v} \in \mathbb{R}^3`, we
+    \in S^3` to a vector :math:`\boldsymbol{v} \in \mathbb{R}^3`, we
     first represent the vector as a quaternion: we set the scalar part to 0 and
     the vector part is exactly the original vector
     :math:`\left(\begin{array}{c}0\\\boldsymbol{v}\end{array}\right) \in