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Non-commutative rings and algebras, non-associative algebras, universal algebra and lattice theory, linear algebra, semigroups. For questions specific to commutative algebra (that is, rings that are assumed both associative and commutative), rather use the tag ac.commutative-algebra.

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
22 votes
4 answers
2k views

Freeness of a Z[x]-module

Definition: Call a mapping $f: \mathbb{Z} \rightarrow \mathbb{Z}$ a generalized polynomial if for any distinct integers $m$ and $n$ we have $(m - n)|(f(m)-f(n))$. It is easy to check that polynomial …
3 votes

Units in a group algebra

Even if $k = \mathbb{F}_p$ for some prime $p$ and $G$ is a $p$-group, it is certainly not easy to describe the group of units of the modular group algebra $kG$. -- For example the Modular Isomorphism …
Stefan Kohl's user avatar
  • 19.6k
13 votes
Accepted

The set of orders of elements in a group

Obviously not for every set $A \subset \mathbb{N}$ there is a group $G$ with $A$ as set of orders of its elements (usually called 'spectrum') -- for example if $G$ has an element of order $n$, then $G …
Stefan Kohl's user avatar
  • 19.6k