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Questions about the properties of vector spaces and linear transformations, including linear systems in general.

1 vote
Accepted

Convergence on iterating a piecewise function

Let $f$ denote the function described in the question. The assertion that every trajectory of $f$ except for the one starting at 0 ends in the cycle -1, 1, -1 is equivalent to the Collatz conjecture s …
Stefan Kohl's user avatar
  • 19.6k
15 votes
1 answer
1k views

Free subgroups of $\mathrm{GL}(2,\mathbb{Z})$

Is there a bound $B$ such that every 2-generator subgroup $G = \langle a, b \rangle \le {\rm GL}(2,\mathbb{Z})$ whose generators do not satisfy a relation of length $\leq B$ is free? If it exists, su …
14 votes
Accepted

Identify one group of linear transformations

If I understand your question right, your group $G$ has order $5160960$, and it has an elementary abelian normal subgroup $N$ of order $2^7$ such that $G/N \cong {\rm S}_8$. This can be found with GA …
Stefan Kohl's user avatar
  • 19.6k
16 votes
0 answers
779 views

How to explain the picturesque patterns in François Brunault's matrix?

How to explain the patterns in the matrix defined in François Brunault's answer to the question Freeness of a Z[x] module depicted below? -- Choosing colors according to the highest power of 2 which …
6 votes
Accepted

Can a block matrix with at least 3 zero blocks of different size on the diagonal and 1's eve...

There are some quantifiers unclear in your question, but regardless of how to read it, your assertion is false. -- The smallest counterexample with blocks of pairwise distinct size all of whose eigen …
Stefan Kohl's user avatar
  • 19.6k
5 votes

Counting matrices over finite fields of a given order

In GAP, you can find the number of elements of order $t$ in ${\rm GL}(2,q)$ by the following function: NumberOfElementsOfGivenOrderInGL2q := function ( q, t ) return Sum(List(Filtered(ConjugacyClas …
Stefan Kohl's user avatar
  • 19.6k
5 votes

Fixed space of the square of a symmetric matrix over $\mathbb{F}_2$

As Geoff Robinson has already said, the answer to the question is no. In dimension $4$, there are in total $120$ counterexamples, of which $96$ have kernel of dimension $1$, and $24$ have kernel of di …
Stefan Kohl's user avatar
  • 19.6k
6 votes

Algorithm for solving systems of linear Diophantine inequalities

GAP provides a function NullspaceIntMat which solves systems of linear diophantine equations. The documentation says: 25.1-2 SolutionIntMat * SolutionIntMat( mat, vec ) ───────────────────────────── …
Stefan Kohl's user avatar
  • 19.6k
22 votes

Small-index subgroups of SL(3,Z)

In order to answer the question we need a finite presentation of ${\rm SL}(3,\mathbb{Z})$ and a general method to find all subgroups of index $\leq n$ of a finitely presented group: A finite present …
Stefan Kohl's user avatar
  • 19.6k