ASKSAGE: Sage Q&A Forum - RSS feedhttps://ask.sagemath.org/questions/Q&A Forum for SageenCopyright Sage, 2010. Some rights reserved under creative commons license.Sun, 25 Aug 2019 20:39:38 +0200Inverse of real sparse matrixhttps://ask.sagemath.org/question/47587/inverse-of-real-sparse-matrix/Dear sagemath community,
I'm a bit surprised in the result that I obtain when I compute the inverse of a real, sparse matrix. The problem occurs in Sagemath 8.7, (on windows).
In my case the matrix is
B=matrix(RR, [[1/6, -1/24, -1/30, 1/120,1/12, 0, 0, 0, 0],
[-1/24,1/60,1/60, 1/420, -1/24, 0, 0, 0, 0],
[-1/30,1/60, 2/105, 1/140, -1/20, 0, 0, 0, 0],
[1/120, 1/420, 1/140, 13/1260, -1/40, 0, 0, 0, 0],
[1/12, -1/24, -1/20, -1/40, 1/3, -1/24, -1/30, 1/120,1/12],
[0, 0, 0, 0, -1/24,1/60,1/60, 1/420, -1/24],
[0, 0, 0, 0, -1/30,1/60, 2/105, 1/140, -1/20],
[0, 0, 0, 0, 1/120, 1/420, 1/140, 13/1260, -1/40],
[0, 0, 0, 0,1/12, -1/24, -1/20, -1/40, 1/6]],sparse=true)
(B.inverse()*B).norm(1)
The result is 138.4999999999923, which is very far from the expected value: 1. If one replaces sparse=true by sparse=false, the result becomes reasonable.The problem also does not occur when I replace RR by QQ.
I suspect that this will not happen in all versions of Sagemath, since I stumbled upon this strange behaviour when I was executing a code that used to work fine years ago. Sadly I do not recall which version I used to have back then. I wanted to ask: what is currently the right way to compute the inverse of a sparse real matrix?
cheers,
IVIVSun, 25 Aug 2019 20:39:38 +0200https://ask.sagemath.org/question/47587/Multiplying sparse matriceshttps://ask.sagemath.org/question/43979/multiplying-sparse-matrices/Can Sage multiply sparse matrices? If so, does it require some special syntax? I've been trying to run the following code:
M = Matrix(QQ, 1000001, 1000001, {(3,2): 27, (2,98): 71})
M2 = M*M
and it hasn't been working. If M is being converted to a dense matrix for multiplication then of course I wouldn't expect this to run. But it shouldn't be hard to do as sparse matrices, right?PWThu, 18 Oct 2018 20:49:55 +0200https://ask.sagemath.org/question/43979/