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Non-commutative rings and algebras, non-associative algebras. Can be used in combination with ra.rings-and-algebras

7 votes
1 answer
201 views

Relative Dickson (trace) criterion for Jacobson radical?

In the following, all algebras are associative and unital. Let $J\left(A\right)$ denote the Jacobson radical of an arbitrary algebra $A$. Recall that this is defined as the set of all $a \in A$ such t …
darij grinberg's user avatar
15 votes
1 answer
2k views

Graded commutativity of cup in Hochschild cohomology

I am trying to get used to Hochschild cohomology of algebras by proving its properties. I am currently trying to show that the cup product is graded-commutative (because I heard this somewhere); howev …
darij grinberg's user avatar
9 votes
2 answers
3k views

Jacobson radical = intersection of all maximal two-sided ideals

I'm embarassed to ask this question, but the literature on noncommutative rings seems to give this a berth as if it was absolutely trivial and not worth discussing, and I can't prove it, so all I can …
darij grinberg's user avatar
26 votes
3 answers
717 views

Subtraction-free identities that hold for rings but not for semirings?

Here is a concrete, if seemingly unmotivated, aspect of the question I am interested in: Question 1. Let $a$ and $b$ be two elements of a (noncommutative) semiring $R$ such that $1+a^3$ and $1+b^3$ a …
darij grinberg's user avatar
6 votes
0 answers
241 views

Proving the spectrum of the Young-Jucys-Murphy elements by formal computation in the degener...

This is really a followup to Why are Jucys-Murphy elements' eigenvalues whole numbers? , specifically to Igor Makhlin's beautiful answer. I'm trying to make it even more beautiful by getting rid of th …
darij grinberg's user avatar
5 votes
0 answers
246 views

Injectivity criterion for surjective coalgebra maps: does it hold in full generality?

Let $\mathbf{k}$ be a commutative ring. Let $C$ be a filtered $\mathbf{k}$-coalgebra. This means a $\mathbf{k}$-coalgebra equipped with an increasing $\mathbf{k}$-module filtration $C^0 \subseteq C^1 …
darij grinberg's user avatar
14 votes
1 answer
540 views

Is the discriminant of a free (as a module) $R$-algebra always congruent to a square modulo 4?

Let $R$ be a commutative ring. Let $A$ be an $R$-algebra (i.e., an $R$-module equipped with an $R$-bilinear multiplication map that turns $A$ into a unital ring). We do not require $A$ to be commutat …
darij grinberg's user avatar
3 votes
0 answers
135 views

Composition of Frobenius $n$-homomorphisms, characteristic-free?

This question is, as so often, a crossbreed of curiosity and laziness. The former has led me to an interesting, but somewhat unsatisfactory paper by Khudaverdian and Voronov (arXiv:2002.02395v2) and, …
darij grinberg's user avatar
8 votes
3 answers
418 views

Smallest faithful representation of an upper-triangular matrix quotient

This is a curiosity question that came out of teaching abstract algebra. Let $F$ be a field, and $n>1$ an integer. Let $F^{n \leq n}$ be the $F$-algebra of all upper-triangular $n\times n$-matrices $\ …
darij grinberg's user avatar
30 votes
4 answers
3k views

A mysterious Heisenberg algebra identity from Sylvester, 1867

I am trying to understand two papers by James Joseph Sylvester: P92: "Note on the properties of the test operators which occur in the calculus of invariants, their derivatives, analogues, and laws of …
darij grinberg's user avatar
3 votes
0 answers
387 views

Software for Combinatorial Algebra sought

I am looking for software which helps me do straightforward tasks in combinatorial algebra. Let me give an example of what I mean by a straightforward task: I have two graded (generally noncommutati …
8 votes
1 answer
345 views

Constructive definition of noncommutative rational functions (aka free skew fields)

The question Let $F$ be a field. (I am fine with assuming $F=\mathbb{Q}$, but I suspect that a "right" answer will be independent of $F$.) Let $k$ be a nonnegative integer. Question. Is there a const …
darij grinberg's user avatar