Intereting Posts

probability question (“birthday paradox”)
Multiple choice question: Let $f$ be an entire function such that $\lim_{|z|\rightarrow\infty}|f(z)|$ = $\infty$.
If $\lim_{n\to \infty}a_n = a\in \mathbb{R}$ . Prove that $\limsup_{n\to \infty}a_n x_n=a\limsup_{n\to \infty}x_n$ .
Uniqueness of memoryless property
What is circumradius $R$ of the great disnub dirhombidodecahedron, or Skilling's figure?
Find all continuous functions from positive reals to positive reals such that $f(x)^2=f(x^2)$
Newton vs Leibniz notation
In a slice category C/A of a category C over a given object A, What is the role of the identity morphism of A in C with respect to C/A
Isomorphism between $\mathbb R^2$ and $\mathbb R$
What are the differences and relations of Haar integrals, Lebesgue integrals, Riemann integrals?
Is computer science a branch of mathematics?
linear dependence proof using subsets
Prove that $C^1()$ with the $C^1$- norm is a Banach Space
If $F$ be a field, then $F$ is a principal ideal domain. Does $F$ have to be necessarily a field?
Factoring multivariate polynomial

I do know that if $m \equiv 1 \pmod 4$ and squarefree, it is probably the discriminant of $\mathbb{Q}[\sqrt{m}]$, and I also know some negative multiples of 27 are discriminants of cubic number fields.

But are there integers that are not the discriminant of any number field? If so, are they listed in Sloane’s?

- Does there exist two non-constant polynomials $f(x),g(x)\in\mathbb Z$ such that for all integers $m,n$, gcd$(f(m),g(n))=1$?
- Vague definitions of ramified, split and inert in a quadratic field
- Where can the original paper by Takagi in English be found?
- Ramification in a tower of extensions
- Coprime cofactors of n'th powers are n'th powers, up to associates, for Gaussian integers
- How to factor the ideal $(65537)$ in $\mathbb Z$?

- Proving $\left|\sqrt2-(a/b)\right|\geq1/(3b^2)$
- Divisibility of discriminants in number field extensions
- Are all algebraic integers with absolute value 1 roots of unity?
- Splitting of a polynomial modulo primes of a ring of integers
- Integral solutions to $y^{2}=x^{3}-1$
- Every ideal of an algebraic number field can be principal in a suitable finite extension field
- How to show that the norm of a fractional ideal is well-defined?
- Linear independence of fractional powers
- Are elementary and generalized hypergeometric functions sufficient to express all algebraic numbers?
- There is a primitive $m^{th}$ root of unity in $\mathbb{Q}_p$ $\Leftrightarrow m \mid (p-1)$

The answer to the title question is no and the answer to the body question is yes.

First, Stickelberger’s theorem asserts that the discriminant $\Delta_K$ of a number field $K$ is congruent to $0, 1 \bmod 4$. So that already rules out half of the possibilities.

Second, Minkowski’s bound implies that if $K$ has degree $n$ then

$$\sqrt{|\Delta_K|} \ge \left( \frac{\pi}{4} \right)^{n/2} \frac{n^n}{n!}.$$

The RHS is always strictly bigger than $1$, so the discriminant can also never be equal to $1$; that is, $\mathbb{Q}$ has no unramified extensions. Moreover it follows that number fields of a given discriminant cannot have arbitrarily high degree, and in fact there are only finitely many number fields of a given discriminant, so for particular small discriminants one can rule them out via casework.

To start, let’s record the value of the Minkowski bound for some small values of $n$.

- For $n = 2$ it is about $1.57$, so a number field of degree at least $2$ has discriminant of absolute value at least $3$.
- For $n = 3$ it is about $3.13$, so a number field of degree at least $3$ has discriminant of absolute value at least $10$.
- For $n = 4$ it is about $6.58$, so a number field of degree at least $4$ has discriminant of absolute value at least $44$.

Let’s also recall that for a squarefree integer $d$ the discriminant of $\mathbb{Q}(\sqrt{d})$ is $d$ if $d \equiv 1 \bmod 4$ and $4d$ otherwise. Now let’s go through the smallest few discriminants in order, skipping the ones that are impossible by Stickelberger to see what Minkowski has to say about them.

- $1$: impossible by Minkowski.
- $-3$: realized uniquely by $\mathbb{Q}(\sqrt{-3})$.
- $4$: impossible by Minkowski (can only be realized by a quadratic field and isn’t).
- $-4$: realized uniquely by $\mathbb{Q}(i)$.
- $5$: realized uniquely by $\mathbb{Q}(\sqrt{5})$.
- $-7$: realized uniquely by $\mathbb{Q}(\sqrt{-7})$.
- $8$: realized uniquely by $\mathbb{Q}(\sqrt{2})$.
- $-8$: realized uniquely by $\mathbb{Q}(\sqrt{-2})$.
- $9$: impossible by Minkowski (can only be realized by a quadratic field and isn’t).
- $-11$: realized by $\mathbb{Q}(\sqrt{-11})$.
- $12$: realized by $\mathbb{Q}(\sqrt{3})$.

$-12$ is the first discriminant whose status I can’t determine from just Stickelberger and Minkowski. If it occurs as a discriminant it must be the discriminant of a cubic field. I think it’s known that in fact the smallest possible discriminant (in absolute value) of a cubic field is $-23$, realized by $\mathbb{Q}(x)/(x^3 – x – 1)$, but I don’t know how to prove this.

In any case, here’s a discriminant I can rule out using an additional technique: $25$ is congruent to $1 \bmod 4$ and, by the Minkowski bound, could only be the discriminant of a cubic field. However, I claim it isn’t the discriminant of a cubic field, and so can’t be a discriminant at all. The reason is that it’s a square, which means that the corresponding cubic field is Galois with Galois group $A_3 \cong C_3$. By the Kronecker-Weber theorem it must therefore be a subfield of the cyclotomic integers $\mathbb{Q}(\zeta_n)$, and it’s known that we should in fact be able to take $n = 5$. However, $\mathbb{Q}(\zeta_5)$ has no cubic subfields since it has degree $4$.

- Change of measure of conditional expectation
- $A$ is normal and nilpotent, show $A=0$
- The use of conjugacy class and centralizer?
- Basic Representation Theory
- Proof that an affine scheme is quasi compact
- Calculate intersection of vector subspace by using gauss-algorithm
- Coin Tossing Game Optimal Strategy Part 2
- Evaluate $ \lim_{(x,y)\to(0,0)} \frac {e^{x+y^2}-1-\sin \left ( x + \frac{y^2}{2} \right )}{x^2+y^2} $
- The limiting case of a discrete probability problem
- Two exercises on characters on Marcus (part 1)
- Partitions of $\mathbb{R}^2$ into disjoint, connected, dense subsets.
- 2 color theorem
- Proving Injectivity
- Are all algebraic integers with absolute value 1 roots of unity?
- Probability distribution of the maximum of random variables