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Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

14 votes
2 answers
8k views

Choosing the algebraic independent elements in Noether's normalization lemma

Given a field $k$ and a finitely generated $k$-algebra $R$ without zero divisors, one knows that there exist $x_1, \ldots, x_n$ algebraically independent such that $R$ is integral over $k[x_1, \ldots, …
user2330's user avatar
  • 1,320
23 votes
6 answers
6k views

Noether's normalization lemma over a ring A

Given a field $k$ and a finitely generated $k$-algebra $R$ without zero divisors, one knows that there exist $x_1, \ldots, x_n$ algebraically independent such that $R$ is integral over $k[x_1, \ldots, …
user2330's user avatar
  • 1,320
12 votes
4 answers
7k views

Definition of étale for rings

Let $A \to B$ be a ring extension. What is the definition of $B/A$ étale ? When $A$ is a field, do we get a nice characterization ?
user2330's user avatar
  • 1,320
29 votes
3 answers
7k views

Non finitely-generated subalgebra of a finitely-generated algebra

Ok, I feel a little bit ashamed by my question. This afternoon in the train, I looked for a counter-example: — $k$ a field — $A$ a finitely generated $k$-algebra — $B$ a $k$-subalgebra of $A$ that …