Add references

doc/source/user_guide/introduction.rst

@@ -6,6 +6,9 @@ Introduction to 3D Rigid Transformations
 Basics
 ------
 
+Following Waldron and Schmiedeler [1]_, we define the basic terms that we use
+throughout this user guide.
+
 .. list-table::
  :widths: 15 35 15 35
 
@@ -53,6 +56,10 @@ its frame) is always expressed with respect to another frame.
 Frame Notation
 --------------
 
+Notation is important for describing and understanding of physical quantities
+in 3D. Furgale [2]_ [3]_ presents one of the most consistent and clear
+approaches and we use it here.
+
 For physical quantities we use the notation :math:`_{A}\boldsymbol{x}_{BC}`,
 where :math:`\boldsymbol{x}` is a physical quantity of frame C with
 respect to frame B expressed in frame A. For example,
@@ -149,8 +156,16 @@ representations on the following pages.
 References
 ----------
 
-1. Waldron, K., Schmiedeler, J. (2008). Kinematics. In: Siciliano, B., Khatib,
+.. [1] Waldron, K., Schmiedeler, J. (2008). Kinematics. In: Siciliano, B., Khatib,
  O. (eds) Springer Handbook of Robotics. Springer, Berlin, Heidelberg.
  https://doi.org/10.1007/978-3-540-30301-5_2
-2. Representing Robot Pose: The good, the bad, and the ugly (slides): http://static.squarespace.com/static/523c5c56e4b0abc2df5e163e/t/53957839e4b05045ad65021d/1402304569659/Workshop+-+Rotations_v102.key.pdf
-3. Representing Robot Pose: The good, the bad, and the ugly (blog): http://paulfurgale.info/news/2014/6/9/representing-robot-pose-the-good-the-bad-and-the-ugly
+
+.. [2] Furgale, P. (2014). Representing Robot Pose: The good, the bad, and the
+ ugly (slides). In What Sucks in Robotics and How to Fix It: Lessons Learned
+ from Building Complex Systems (ICRA workshop). Note: these slides seem to be
+ lost, but the blog below conveys the same message.
+ http://static.squarespace.com/static/523c5c56e4b0abc2df5e163e/t/53957839e4b05045ad65021d/1402304569659/Workshop+-+Rotations_v102.key.pdf
+
+.. [3] Furgale, P. (2014). Representing Robot Pose: The good, the bad, and the
+ ugly (blog).
+ https://web.archive.org/web/20220420162355/http://paulfurgale.info/news/2014/6/9/representing-robot-pose-the-good-the-bad-and-the-ugly