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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

47 votes
3 answers
5k views

Class Numbers and 163

This is a bit fluffier of a question than I usually aim for, so apologies in advance if this doesn't pass the smell test for suitability. Likely my favorite fun fact in all of number theory is the jux …
73 votes
2 answers
10k views

Please check my 6-line proof of Fermat's Last Theorem.

Kidding, kidding. But I do have a question about an $n$-line outline of a proof of the first case of FLT, with $n$ relatively small. Here's a result of Eichler (remark after Theorem 6.23 in Washingt …
8 votes

Gauss linking integral and quadratic reciprocity

This is a beautiful aspect of the analogy. In fact, I think it's important and cool to view it as a consequence of previously-established aspects of the analogy, rather than a new analogy-by-fiat. U …
Cam McLeman's user avatar
  • 8,467
31 votes
Accepted

Why is $(\mathbb{Z}/3\mathbb{Z})^3$ not a class group of an imaginary quadratic number field ?

The only proof that I know that $(\mathbb{Z}/3\mathbb{Z})^3$ does not appear as the class group of a quadratic imaginary number field is by brute force search. Roughly, the idea is that since class n …
Community's user avatar
  • 1
48 votes
6 answers
5k views

Algebraic Attacks on the Odd Perfect Number Problem

The odd perfect number problem likely needs no introduction. Recent progress (where by recent I mean roughly the last two centuries) seems to have focused on providing restrictions on an odd perfect …
11 votes
Accepted

sums of rational squares

This result is pretty shy of needing the full Hasse-Minkowski Theorem. Indeed, since Fermat already knew which integers were a sum of two integer squares, it would suffice for him to show that those …
Cam McLeman's user avatar
  • 8,467
2 votes

relations between class numbers of quadratic extensions

The short answer to your question is basically no, there's essentially no connection between the prime powers $q^i$ dividing $h_p$ and $h_{-p}$. It's true that there's a general relationship betwee …
Cam McLeman's user avatar
  • 8,467
15 votes
Accepted

Unramified extensions of number fields

Two things: 1) Yes, certainly. By class field theory and the finiteness of the class group, the maximal abelian unramified extension of any number field is of finite degree. Thus any infinite un …
Cam McLeman's user avatar
  • 8,467
12 votes

Where can I find a modern write-up of Heegner's solution of Gauss' class number 1 problem?

In his article On the "gap'' in a theorem of Heegner, Stark does a pretty thorough job of explaining where people thought the purported gap came from, to what extent it actually was a gap, and what yo …
Cam McLeman's user avatar
  • 8,467
10 votes
Accepted

Explicitly describable maximal unramified extension of a number field

No, I'm pretty sure not. In general, the theory is much more developed for the maximal pro-p-quotient of the groups you're asking about, and even in this more explored setting, not a single explicit …
Cam McLeman's user avatar
  • 8,467
13 votes

Does $\pi_1(Spec(\mathbb{Z}[1/p]))$ depend on p?

To add on to Pete's answer, let me comment that the differences are even more pronounced if we look at the maximal pro-$p$ quotient $\pi_1(\operatorname{Spec}(\mathbb{Z}\left[\frac{1}{p_1p_2\cdots p_r …
Cam McLeman's user avatar
  • 8,467
16 votes

number fields with no unramified extensions?

The question itself is certainly still open. Mostly as an exercise for myself, I'll coalesce my comments above into an answer, and add in some details about where various pieces of the philosophy com …
Cam McLeman's user avatar
  • 8,467
10 votes

A coverage question

It is very likely that every (positive) odd number is covered by a sum of this type. As Robin Chapman points out, this is equivalent to asking, for a given odd number $h$, whether there exists an odd …
Cam McLeman's user avatar
  • 8,467
5 votes
Accepted

Congruences mod primes in Galois extensions

Sure. $a\equiv b\pmod{\mathfrak{P}}$ just means $a-b\in\mathfrak{P}$. Taking norms to any subfield $K$ of $\mathbb{Q}(\zeta_n)$ (e.g., $\mathbb{Q}$ or $\mathbb{Q}(\zeta_m)$) gives you $N_{\mathbb{Q} …
Cam McLeman's user avatar
  • 8,467
4 votes

Algebraic integers on the unit circle

"Do these objects have a name" Probably not. "Multiplicative subgroup of $S^1$ generated by algebraic integers" is pretty descriptive, and not of such fundamental importance that it's worth shorteni …
Cam McLeman's user avatar
  • 8,467

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