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

3 votes
Accepted

Classification of commutative ring ideal closure operators?

Note that for any such "closure operations" one has $cl(A) = cl(\langle A\rangle)$ where $\langle A\rangle$ denotes the ideal generated by $A$. Hence it can be defined as an operation on ideals. Now …
Simon Henry's user avatar
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10 votes

Why we can analytically define $ε$ in dual numbers so to distinguish $ε$ from $-ε$ but canno...

The short answer is "because you are considering $\mathbb{C}$ and $\mathbb{R}[\epsilon]$ them with different structure", which is an artificial choice. Maybe to illustrate the point : If I want to wor …
Simon Henry's user avatar
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3 votes
Accepted

Matrix diagonalization and eigenvector computation constructively

The following works constructively over an arbitrary local ring $R$ (constructively, $\mathbb{R}$ is a local ring). Assume that you matrix $M$ is canceled by a polynomial $Q$, of degree $m$ (with lea …
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6 votes
Accepted

Free augmented algebras

For any choice of $\lambda_1,\dots,\lambda_n$ there is an isomorphism: $$ k[X_1^{[\lambda_1]},\dots,X_n^{[\lambda_n]} ] \simeq k[Y_1^{[0]},\dots,Y_n^{[0]} ] $$ Which is given by $X_i \leftrightarro …
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7 votes
1 answer
233 views

Functions on Stone spaces as "enveloping algebra" of Boolean algebra

I'm looking for references for the following closely related facts: Given a Boolean algebra $B$, I denote by $\mathbb{Z}[B]$ the free ring generated by symbols $e_b$ such that $e_b e_{b'} = e_{b \cap …
Simon Henry's user avatar
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8 votes

How to construct a constructive proof from a non-constructive proof using prime ideals?

Here is a method which is very efficient in the case were "constructive" is interpreted as "no axiom of choice at all, not even countable and no law of excluded middle", i.e. essentially "topos logic" …
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12 votes

Does the category of local rings with residue field $F$ have an initial object?

I will show that this is not the case for $F=\mathbb{F}_9$. The proof generalize to any $\mathbb{F}_{p^k}$, with $k >1$. I'm starting from the observation that: $\mathbb{F}_9 \simeq \mathbb{Z}[i]/(3) …
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20 votes
Accepted

What is a module over a Boolean ring?

Theorem: Given $A$ a boolean ring/boolean algebra then there is an equivalence of categories between the category of $A$-modules and the category of sheaves of $\mathbb{F}_2$-vector spaces on Spec $A$ …
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