10.15.3 Can you answer these questions?

10.15.3.1 Double pendulum with symplectic Runge-Kutta option for NDSolve not working

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=302858

I would like to use the Symplectic Partitioned Runge-Kutta integrator from Mathematica to solve the nonlinear double pendulum equations. 
I set up the problem by first calculating the Hamiltonian ... 
[differential-equations]
 

asked by Meclassic https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=5476 Score of 1

10.15.3.2 Connection to MongoDB on Atlas: Timeout calling ismaster

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=302800

The problem 
How to access my MongoDB on Atlas from a Wolfram Notebook? 
Following the Wolfram doc, I try this: 
... 
[wolfram-cloud] [mongodb]
                                                                                  

                                                                                  
 

asked by Joseph Marie https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=98441 Score of 3

10.15.3.3 Enclosing ParametricPlot3D to create a solid volume

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=302725

I'm creating a 3D parametric hourglass shape that I hope to fill, making the center solid to use as a 3D volume, because I can't set parameters to it in COMSOL for simulations right now. I know that a ... 
[plotting] [regions] [parametric-functions] [discretization]
 

asked by Char https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=66040 Score of 1