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Algebraic number fields, Algebraic integers, Arithmetic Geometry, Elliptic Curves, Function fields, Local fields, Arithmetic groups, Automorphic forms, zeta functions, $L$-functions, Quadratic forms, Quaternion algebras, Homogenous forms, Class groups, Units, Galois theory, Group cohomology, Étale cohomology, Motives, Class field theory, Iwasawa theory, Modular curves, Shimura varieties, Jacobian varieties, Moduli spaces

2 votes

Is there a purely algebraic criterion which characterizes the real algebraic numbers?

As other correspondents have pointed out there is no algebraic way to distinguish the elements of $\mathbb{Q}^{alg}\cap\mathbb{R}$ but there is an algebraic way of distinguishing totally real algebrai …
Robin Chapman's user avatar
31 votes

Which number fields are monogenic? and related questions

To add to Keith's answer, there are various classes of number fields which are known to be not monogenic. For instance, the following paper Marie-Nicole Gras, Non monogénéité de l'anneau des entiers …
Robin Chapman's user avatar
1 vote

What are Mean Values of Ideal Densities in Galois Extensions?

This may be a reference to the the Davenport-Heilbronn theorem on the distribution of discriminants of number fields. See math.stanford.edu/~fthorne/davenport-heilbronn.pdf for a nice exposition. Str …
Robin Chapman's user avatar
5 votes

Is there a notion of Galois extension for Z / p^2?

There's the notion of Galois ring. Let $K$ be the degree $m$ unramified extension of $\mathbb{Q}_p$ and let $\mathcal{O}_K$ be its ring of integers. Then the quotient $R=\mathcal{O}_K/p^n\mathcal{O}_ …
Robin Chapman's user avatar
20 votes

$A_5$-extension of number fields unramified everywhere

Here's the standard example. I found it in Lang's Algebraic Number Theory where he attributes it to Artin. Let $K$ be the splitting field of $X^5-X+1$ over $\mathbb{Q}$. Then $K$ has Galois group $S_5 …
Robin Chapman's user avatar