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

11 votes

Different definitions of the dimension of an algebra

In non-commutative algebra Krull dimension has been generalised by Gabriel & Rentschler. A decent account of it can be found in Chapter 6 of McConnell and Robson's book on non-commutative Noetherian r …
Simon Wadsley's user avatar
8 votes
1 answer
1k views

Primes in a (commutative) Jacobson ring

Recall that a commutative ring is Jacobson if every prime ideal is the intersection of the maximal ideals that contain it. In the exercises of a commutative algebra course I gave I asked the student …
Simon Wadsley's user avatar
8 votes
3 answers
907 views

Generic Noether normalisation

Suppose that $M$ is a finitely generated module over $A=k[X_1,\ldots,X_n]$ of Krull dimension $m$ with $k$ an infinite field. Then one version of Noether normalisation says there is an $m$-dimensional …
Simon Wadsley's user avatar
7 votes

Different definitions of the dimension of an algebra

Often the most useful dimension in non-commutative algebra is the length of the minimal injective resolution of the ring as a module over itself. In many important cases this is the same as the global …
Simon Wadsley's user avatar
6 votes

Maximal Ideals in Formal Laurent Series Rings?

I think that the question difficult as illustrated by Hailong's answer. I suspect that it will be hard to even find a nice parameterisation of the $H$-orbits of maximal ideals in your refined question …
Simon Wadsley's user avatar
3 votes
Accepted

Is the pair $(C([0 \;1]),\mathbb{C})$ a consecutive pair?

The answers to the question prime ideals in C([0,1]) explain why the answer is no, assuming that Yemon Choi was mistaken in his answer in believing that you are only interesting in continuous homomorp …
Simon Wadsley's user avatar
3 votes

Generic Noether normalisation

In case anyone else has the same question and discovers this page I have just found a more explicit reference for this result: Remark 3.4.4 of A Singular introduction to commutative algebra by Greuel …
Simon Wadsley's user avatar
3 votes
1 answer
922 views

How exotic can DVRs be in the ring of rational functions over a local field?

Suppose that $R$ is a complete DVR with field of fractions $K$, uniformiser $\pi$ and residue field $k$. Let $B$ be a subring of the ring $K(t)$ of rational functions over $K$. Moreover assume that $ …
Simon Wadsley's user avatar
2 votes

A ring such that all projectives are stably free but not all projectives are free?

Example 1.2.2 in Chapter 1 of Weibel's book in progress on K-theory http://www.math.rutgers.edu/~weibel/Kbook.html says that $R_2$ in the notation of your question has a stably free module that is not …
Simon Wadsley's user avatar