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2019-06-05 19:35:00 -0500 | commented answer | Bel decomposition Thanks, I understand. |

2019-05-22 13:44:30 -0500 | commented answer | Bel decomposition Thank you very much! I've put BEFORE you code the following (Schwarzschild metric): I do not understand what is e0 in you code, so I do not know how to proceed. Thanks, Daniel |

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2019-05-18 08:14:37 -0500 | asked a question | Bel decomposition Hi All, Is there any way to obtain the Bel decomposition (https://en.wikipedia.org/wiki/Bel_dec...) into the electrogravitic, magnetogravitic and topogravitic tensors given the metric? Thanks, Daniel |

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2019-04-14 03:08:34 -0500 | commented answer | multiprocessing.Pool does not work In Windows it is broken too! |

2019-04-11 21:12:18 -0500 | marked best answer | Differential forms best package I understand there are two different ways to use differential forms in SageMath: 1) in the reference document "manifold.pdf" sage: M = Manifold(4, 'M') sage: a = M.one_form('A') or F = M.diff_form(2, 'F') 2) in the reference document "tensors.pdf" sage: x, y, z = var('x, y, z') sage: U = CoordinatePatch((x, y, z)) sage: F = DifferentialForms(U) sage: form1 = DifferentialForm(F, 0, sin(x*y)); form1 Which is the best? Are they compatible? Can I mix and match from both packages? What are their pros and cons? Thanks. |

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2019-02-27 11:08:30 -0500 | commented answer | No output when documentation say it should be Thanks, I understand. |

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2019-02-09 01:40:47 -0500 | commented question | No output when documentation say it should be Cont. No output |

2019-02-09 01:37:47 -0500 | commented question | No output when documentation say it should be Sage version is 8.4, Windows Native on a 64bit machine, installed from SageMath-8.4-Installer-v0.4.1.exe I did not compile Sage by myself. I use Jupyter Notebook. I don't use the command line. I did not get any errors. I just got no output at all (more) |

2019-02-08 13:36:30 -0500 | asked a question | No output when documentation say it should be Hi all, Consider http://doc.sagemath.org/html/en/refer... The very last command of this web page is The output of this command should be: True, True, True, but I get no output at all.
In order to duplicate you should include also the previous commands beginning from the Daniel |

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2019-01-12 10:52:38 -0500 | answered a question | Combine plots with built-in Maxima, trajectory in Sage available? You can combine plots in Maxima with the command "xfun" inside plotdf e.g. In you case: you can use several functions inside the quotes, separated by semi colons. Daniel |

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2018-12-16 01:56:13 -0500 | commented question | Velocity of time I am using Windows 10 on a 64bit machine with SageMath native. Just to clarify, I can't see the 3D plots in the Jupyter nbviewer rendition of the file. If I download the ipynb file to my machine, change the 3D viewer to 'tachyon' then I can see the 3D plots. I'm using Microsoft Edge to open Jupyter notebooks with no problems. As I reported a while ago, the only 3D viewer that works on Windows 10 is 'tachyon'. Daniel |

2018-12-16 01:38:32 -0500 | commented question | Velocity of time Microsoft Edge. |

2018-12-16 01:36:54 -0500 | marked best answer | Wrong Pointer Hi All, When you open:
http://nbviewer.jupyter.org/github/sa...
and try to download the corresponding ipynb file using the link pointed at by Please fix that. Thanks, Daniel |

2018-12-14 14:27:23 -0500 | commented answer | Velocity of time Cont. The big question is Dt_0. Is this something the sensor system should measure, imposed on my by the outside conditions? I understand from your explanation that there is another constraint: the fact that the norm of the tangent vector should be constant to preserve a time-like trajectory, and from this constraint I can calculate Dt_0 using the other measured values. I have several Maxima files in which I calculate numerically the trajectory of a mass in different geometries using rkf45. I always wondered what value should I select for Dt_0. I usually select 1. Thanks, Daniel |

2018-12-14 14:21:35 -0500 | commented answer | Velocity of time Cont. In order to do that, the navigation system needs the initial conditions. I understand t_0=0, this is arbitrary, I understand (r_0, th_0, ph_0), my current radial distance, its polar angle, and the azimuth angle, those are values that should be measured by some kind of on board sensor system, based on information from the outside. I don't know how it is measured but I guess it can be done. I understand (Dr_0, Dth_0, Dph_0), my current radial velocity, and polar and azimuthal angular velocities, I also guess those can be measured too. So (r_0, th_0, ph_0) and (Dr_0, Dth_0, Dph_0) are measured, that is, imposed on me, based on information from the outside and referred to the selected coordinate system. |

2018-12-14 14:18:58 -0500 | commented answer | Velocity of time Hi Karim, Thank you very much for taking the time to answer this question so thoroughly, I really appreciate it. If it is not much of an imposition, I'll recommend including this explanation in the file itself. Sometimes I imagine being in a spaceship, the screen showing a big Black Hole and me wandering if I am going to crash into the Black Hole or I will be able to acquire a stable orbit. I guess the on board navigation system is able to choose the easiest Boyer-Lindquist coordinate system (it is a Schwarzschild Black Hole, no rotation and no charge) and calculate my trajectory. |

2018-12-11 08:00:27 -0500 | asked a question | Velocity of time Hi All, I was looking at http://nbviewer.jupyter.org/github/sa... at the paragraph Computing numerical solutions/Timelike geodesics/Bounded orbit, at the following command: I understand there are 4-coordinates (t,r,th,ph), and for each one there is a 2nd order Geodesic equation, so in total should be 8 initial conditions. How did you choose the value of Dt_0? How did you calculate this value? Is this the velocity of time at time zero. What is the physical significance of this value? P.D. Did you notice there are no 3D figures in this Notebook? Thanks, Daniel |

2018-12-11 03:19:04 -0500 | marked best answer | set_name causes an error on external_derivative display I have the following code: I get a If I delete the Daniel |

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