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Jendrik Stelzner's user avatar
Jendrik Stelzner's user avatar
Jendrik Stelzner's user avatar
Jendrik Stelzner
  • Member for 8 years, 4 months
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13 votes
0 answers
232 views

Is $\mathrm{U}(\mathfrak{a} \oplus \mathfrak{b}) \cong \mathrm{U}(\mathfrak{a}) \otimes \mathrm{U}(\mathfrak{b})$ over a commutative ring?

9 votes
1 answer
1k views

Thinking about the group algebra $k[G]$ as functions on $G$

7 votes
1 answer
675 views

Proof of Maschke’s theorem: Why is $\hat{p}$ again a projection?

7 votes
2 answers
439 views

Intuition for the invariance lemma

6 votes
1 answer
568 views

Simultaneous diagonalizability of infinitely many endomorphisms in infinite dimension

6 votes
2 answers
828 views

Interpreting a chain map as a double complex (Weibel Exercise 1.2.8)

6 votes
3 answers
133 views

Are the elements of a Lie algebra separated by its finite-dimensional representations?

6 votes
1 answer
87 views

Relatively compact subsets in $\mathsf{Top}(X, Y)$ for the product topology (Exercise 2.19 in “Topology: A Categorical Approach”)

5 votes
1 answer
256 views

Ore extensions of noetherian $k$-algebras are again noetherian (proof explanation)

4 votes
1 answer
50 views

Concluding that $\mathcal{C}/S$ is complete if $\mathcal{C}$ is complete

4 votes
2 answers
224 views

Reference Request: Differential Graded Hopf Algebras

4 votes
1 answer
253 views

An inner product for characters of modules over arbitrary fields

4 votes
2 answers
222 views

Why is the spectrum of a commutative ring only concerned with fields?

4 votes
0 answers
110 views

Exercise 2.2.12 in Topology and Groupoids

4 votes
1 answer
165 views

Is there a notion of $\mathfrak{g}$-$\mathfrak{h}$-bimodules?

3 votes
0 answers
57 views

Correct terminology for “commutative coproducts” and how to formalize them.

3 votes
1 answer
237 views

Simultaneous semisimplicity of commuting endomorphisms

3 votes
1 answer
93 views

Is $\operatorname{Hom}(E,M)$ simple as an $\operatorname{End}(M)$-module if $E$ is simple?

3 votes
1 answer
99 views

A generalization of the alternating property of a Lie bracket

3 votes
0 answers
171 views

Mistake in the proof of Theorem 2.24 of Quiver Representations by Ralf Schiffler?

3 votes
1 answer
114 views

Lang's proof of the uniqueness of multiplicities for semisimple modules

2 votes
0 answers
777 views

Why do characters span the space of class functions?

2 votes
1 answer
175 views

Right derived functors of composite

2 votes
1 answer
298 views

Why is the Chevalley-Eilenberg differential a coderivation?

2 votes
0 answers
110 views

Checking the antipode for a dg-Hopf algebra on algebra generators

2 votes
0 answers
42 views

Reference Request: Proof of $\mathrm{H}(\mathrm{Prim}\,\mathcal{H}) \cong \mathrm{Prim}\,\mathrm{H}(\mathcal{H})$ for cocommutative dg-Hopf algebras

2 votes
0 answers
52 views

Unnecessary condition in the definition of an isomorphism of root systems

2 votes
2 answers
77 views

Relation between $n$ and $m$ with $\mathfrak{g}^{(n)} = \mathfrak{g}^{(n+1)}$ and $\mathrm{rad}(\mathfrak{g})^{(m)} = 0$

2 votes
0 answers
105 views

Connection between uniqueness of Jordan normal form and Jordan–Chevalley decomposition

2 votes
1 answer
65 views

How is a Lie algebra called whose derived series vanishes after infinitely many steps?