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

3 votes

Generalization of Weak Nullstellensatz?

Or see Proposition 2.4.6 in Bjorn Poonen's book Rational Points on Varieties (link). This is almost exactly the result you conjectured, just a bit more general: Let $X$ be a $k$-variety. Then the map …
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  • 4,745
7 votes

Set of primes $p$ such that $\mathrm{Hom}(A, \mathbb{F}_p)=\emptyset$

Here's a sketch of an answer. I think the answer is that you can get three types of sets: (i) finite sets, (ii) co-finite sets, and (iii) sets of the form $$ S_f = \{ p : f(x) ~ \textrm{has a root in …
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  • 4,745
4 votes

On the fixed point of automorphism of $\mathbb F_3[[T]]$

Let's write $R$ for $\mathbb{F}_3[[T]]$ and $K$ for its fraction field. Suppose $t \in R \setminus \mathbb{F}_3$ satisfies $\sigma(t)=t$. I claim that $\sigma$ then has finite order. Suppose first tha …
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  • 4,745
4 votes

Pythagorean number in Artin's theorem on nonnegative rational fractions

Apparently, we have $N \leq 2^n$. See these slides by Jean-Louis Colliot-Thélène, belonging to a lecture he gave in Leiden in 2011: http://www.math.u-psud.fr/~colliot/Kloostermanlezing.pdf First, he w …
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1 vote

How to show an invariant subfield of rational function field $\mathbb{Q}(x)$ under a certain...

The answer as to the surjectivity of $\alpha$ is no. As in algebraic number theory, the simplest way to prove that an element is not a norm is by local considerations. Let us consider $$ y=\frac{(x^3- …