At the bottom of the following web site are the 4 equations that I am using for a double pendulum simulation. http://scienceworld.wolfram.com/physics/DoublePendulum.html The equation for C2 has the variables P1 and P2 and I can't work out what they are. Is it a typo and they are meant to be Pθ1 and Pθ2? Any help would be much appreciated.