ASKSAGE: Sage Q&A Forum - Latest question feedhttp://ask.sagemath.org/questions/Q&A Forum for SageenCopyright Sage, 2010. Some rights reserved under creative commons license.Tue, 04 Dec 2018 02:18:37 -0600Sagemanifold - Connection components from a tensor (not a metric)http://ask.sagemath.org/question/44572/sagemanifold-connection-components-from-a-tensor-not-a-metric/ Dear community.
This might sound *dump*, but I'm trying to determine whether a tensor satisfy the properties of a metric (under certain conditions). Of course it is a (0,2)-symmetric tensor, call it $S$, but I cannot (to my understanding) calculate the (Levi-Civita-like) connection components that would be associated to $S$... unless I declare it as a metric.
The way it is implemented makes sense... and it's solid!
## What I did...?
I defined like a metric and calculate the associated connection (and curvatures)
## Why should I do something else?
In the file `src/sage/manifolds/differentiable/metric.py` the metric is defined (as it should) to be symmetric, but it does not allow to consider extensions of General Relativity like say Einstein--Strauss model. Thus, I need an instance to calculate the *derived quantities* of a tensor that is a generalization of a metric.
### Question:
**Is this possible?**DoxTue, 04 Dec 2018 02:18:37 -0600http://ask.sagemath.org/question/44572/Covariant Derivative gives Error, why? (SAGE 7.5.1)http://ask.sagemath.org/question/36777/covariant-derivative-gives-error-why-sage-751/This simple code gives an Error:
f = function('f')
B=Manifold(2,'B',start_index=1)
polar.<R,Phi> = B.chart(R'R:(0,+oo) Phi:(0,2*pi):\Phi')
G = B.riemannian_metric('G')
G[1,1]=diff(f(R),R)
G[2,2]=f(R)^2
nabla=G.connection()
S=B.tensor_field(1,1)
S[1,1]=R^(0.5)
S[2,2]=R^3
S.display()
nabla(S)
Error: TypeError: unable to convert R to an integer
This Error doesn't occur either if I avoid the square root in S[1,1]=R^(0.5), for example by writing S[1,1]=R^(0.4) or if I avoid the dependence of the derivative of f in the first entry of the metric, for example by writing G[1,1]=f(R)^2.
This Error also doesn't occur in older Sage versions, for example Sage 7.1
Thanks a lot for help!ClaudiaWed, 01 Mar 2017 09:28:32 -0600http://ask.sagemath.org/question/36777/Using differential forms---within SageManifoldshttp://ask.sagemath.org/question/34321/using-differential-forms-within-sagemanifolds/ Hi all.
I'm aware of the implementation of `DifferentialForms` within `SageManifolds`, but I'd like to know how could I use this forms with ease.
In the **Sage Reference Manual_Manifolds**, there are examples of `AffineConnection` and the `connection_form`. However, it seems that the last (`connection_form`) does not allow to store the calculations, like for example:
nab = g.connection() ## This works for the usual connection
nab.display()
omega = nab.connection_form() ## DOES NOT work, one needs to specify components
I would like to calculate all the components of the connection form, to be able of compute *covariant exterior derivatives* of other objects.
How can the connection, curvature and torsion forms be stored (as differential forms)?
Thank you, and cheers.DoxWed, 03 Aug 2016 10:57:44 -0500http://ask.sagemath.org/question/34321/Can't source data from web using R scripts?http://ask.sagemath.org/question/25702/cant-source-data-from-web-using-r-scripts/Hi,
I'm attempting to source dynamic data from the web (through R within sage cloud terminal, package httr). But every attempt, for any website returns in a "Failed to connect to *url* port 80: Connection refused", where URL could be really any web resource, e.g. an xml file accessible on the web
Is my session behind a firewall? Can I amend those settings then?SarangWed, 04 Feb 2015 05:31:37 -0600http://ask.sagemath.org/question/25702/Sagevmware4.6 internet connection problemhttp://ask.sagemath.org/question/7999/sagevmware46-internet-connection-problem/Hi,
I have just installed sagevmware4.6 in VMWare Player on my PC at home (Direct Internet Connection) and on my PC at my workplace (Internet Connection thru a proxy with authentication) on Windows 7 platforms. Unfortunately I couldn't connect to the internet neither using the web browser nor the terminal.
I appreciate your help.
MSBboudellTue, 15 Mar 2011 02:59:33 -0500http://ask.sagemath.org/question/7999/