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20190907 13:42:37 +0100  commented answer  Finitely presented group simplification Sorry for the late reply, it seems like your answer works perfectly, thanks!! 
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20190828 17:26:11 +0100  commented answer  Finitely presented group simplification It might be that the previous example I gave was wrong (still unsure), but the following one has 17 elements (so, it's necessarily Z/17Z), but still it can't seem to be reducible.

20190828 12:25:45 +0100  asked a question  Finitely presented group simplification Hi! I have a bunch of finitely presented groups, with many generators and relations. I know that all of these are in fact cyclic groups, but many times using the "simplified()" function, I get a simpler presentation with 2 generators, rather than only one. The following is one example: H is in fact just Z/27Z. Is there another way to simplify these presentations, in order to get a minimal one? The problem here is that I do not only need to identify the specific group, but also to recover the image of the previous generators in the simplified one (as done by the simplification_isomorphism() function). 
20190724 11:30:33 +0100  asked a question  Crossings in knot diagram Hi! Given a plot of a knot, I was wondering if it is possible to obtain the position (within the plot) of its crossings, in order to then add some decorations to the diagram. 
20190522 22:55:45 +0100  commented question  Formal determinant of symbolic matrix the question seems something like: how many nonzero terms are there in the determinant, if we don't allow for cancellations? 
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20190522 22:29:15 +0100  commented question  Formal determinant of symbolic matrix the properties asked do not seem to be related to the noncommutativity of variables if I guess correctly 
20190522 22:27:53 +0100  commented answer  Formal determinant of symbolic matrix I had a similar problem: one needs to keep track of the nonzero monomials appearing, so if there's cancellations it does not work 
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20190522 16:32:20 +0100  asked a question  Formal determinant of symbolic matrix I have some sparse symbolic matrices, and want to compute their formal determinant (without cancellation of terms). In other words, if I have the matrix I would like the result of to be xy  xy, rather than just 0. The variables in each monomial are allowed to commute with each other, but on the other hand I would like all monomials containing a 0 to vanish (i.e if in the example above M = Matrix(SR, [[x,0],[x,y]])), then the determinant should be just xy, rather than xy  0*x). Is there a way to achieve this (without using the expansion of the determinant as permutations, since the dimension of the matrices gets quite big!)? Thanks in advance! 
20190112 15:47:54 +0100  commented answer  braid closures Thanks, that works perfectly! 
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20190111 13:45:51 +0100  asked a question  braid closures Hi! I've noticed that when taking the closure of a braid on $n$ strands, the resulting link is obtained by ignoring any "unused" strand: outputs 2 1, instead of 2 2. In the documentation for the braid group I've read "The behavior of removing unused strands from an element of a braid group may change without notice in the future. Do not rely on this feature." Is there a workaround that allows to have the same number of components in a braid and in its closure? Note that braids given as an array of generators (as in the example above) are automatically simplified, so closing the braid [2,2] (which should give the $2$component unlink) produces the unknot. (In particular, the empty braid on $n$ strands should close to the $n$component unlink.) 
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20181124 14:59:55 +0100  commented answer  Free/Braid groups not working that worked!! thanks a lot! 
20181121 15:34:23 +0100  commented question  Free/Braid groups not working Any chance this could be solved easily? or should I just downgrade both ubuntu and sage? 
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20181105 15:18:20 +0100  commented question  Free/Braid groups not working Got `Listing... Done sagemath/bionic,now 8.17ubuntu1 amd64 [installed]`" 
20181102 14:42:22 +0100  commented question  Free/Braid groups not working Ubuntu 18.04, I am on a university machine, so I have no idea of how it was installed! 
20181102 12:05:55 +0100  asked a question  Free/Braid groups not working Hi, I am working with SageMath 8.1, and it seems like "FreeGroup()" (and as a consequence BraidGroup() ) are not working properly. If I write in the notebook I get the following: Am I doing something wrong? Thanks! 
20180412 16:34:59 +0100  commented question  connected sum of knots so it does not work either way? the result should be the same! anyway, it seems to have problems even when all knots are called in different ways, very weird! 
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20180411 13:14:13 +0100  asked a question  connected sum of knots Hi! I was trying to recursively construct connected sums of knots, but I seem to run in to some problems when connect summing a knot to itself more than two times:
This does not work:

20180326 19:40:20 +0100  commented question  problems with gauss code for knots It seems that the braid function is not working properly 
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20171107 13:34:58 +0100  asked a question  different results for Alexander polynomials Hi! I just noticed that if one computes the Alexander polynomial of a knot, one gets different results according to whether the knot is presented as the closure of a braid or as a knot: t = var('t') B = BraidGroup(2) b = B([1,1,1]) knot = Knot(b) print b.alexander_polynomial() print knot.alexander_polynomial() Output: t^2  t^1 + 1 t^1  1 + t Of course the polynomials differ only by a t^n multiplication, but I guess it would be better if the two coincided right away. My question is: is this issue going to be solved in the future? (I can live happily either way, just wanted to avoid modifying some stuff I'm working on!) Thanks! 