Welcome to MathOverflow

Question

9

3

If $K/\mathbb{Q}$ is a number field which is not $\mathbb{Q}$ or a quadratic imaginary field then, by the Dirichlet unit theorem it has a unit of infinite order. Is there a simple proof of this fact which doesn't haul in all the machinery of the unit theorem? In a related question is it true that such a $K$ is always generated over $\mathbb{Q}$ by one unit (which might not be of infinite order -- cf. the case of a full cyclotomic field), and if it is, is there a simple proof of this fact?

flag	asked Feb 14 2010 at 15:17 Victor Miller 3,395●2●14●26

1

Nice question. I have thinking about something somewhat related lately: every f.g. domain has a f.g. unit group. Everywhere I look, the proof uses the Dirichlet unit theorem. But it doesn't feel like that should be necessary. – Steve D Feb 14 2010 at 15:29

1

I guess that what you're expecting is a (mildly) simple formula for a unit of infinite order. Even in the easiest (quadratic) case, AFAIK there is no closed-form formula in terms of d>0 for x(d),y(d) such that x(d)^2-dy(d)^2=1 (and y(d) nonzero), so it seems rather unlikely. – Ewan Delanoy Feb 14 2010 at 15:50

@Ewan: I realized that there probably couldn't be a simple formula because of the Pell's equation case you mentioned, but still thought that there might be a simpler proof, since the unit theorem proves existence of $r_1 + r_2 - 1$ independent units. – Victor Miller Feb 14 2010 at 15:53

1

I've been thinking about the case of real abelian fields. In the case of the real subfield of $\mathbb{Q}(\zeta_n)$ we do have an explicit formula for the cyclotomic units, and the action of the Galois group on them is completely explicit. So to show the existence of a non-trivial unit in a subfield we just need to show that the things fixed by a the corresponding subgroup of galois is non-trivial. This would seem to be a linear algebra problem over finite $\mathbb{Z}$-modules. – Victor Miller Feb 14 2010 at 17:56

2

If L/K is non-trivial, the unit rank r(L) is greater than r(K) unless L/K is CM. Since Z^n cannot be covered by the union of a finite number of subgroups of the form Z^m with m < n, there exists a unit in L that does not lie in any subfield (if L is not CM). If L is CM, then the index [O^*_L,O^*_K] = I is finite. If I=1 then no unit generates L. If I > 1, let x be any unit in L that is not in K. Since the cosets of I*(Z^n) in Z^n cannot be covered by the images of Z^m for m < n, there exists a unit y in K such that (xy)^m generates K; then xy generates L. (If K=Q and L=IQF the result is clear) – Lavender Honey Feb 14 2010 at 18:19

show 2 more comments

Hunter Brooks · Answer 1 · 2010-02-14 15:57:35Z UTC

The answer to question 2 as stated is no. Let $K = \mathbb{Q}(\sqrt{p},\sqrt{-b})$, with $p$ 1 mod 4. By Frohlich and Taylor, p. 196, the fundamental unit group is generated by a unit from the real quadratic subfield. It then suffices to show that we can pick b so that K contains no roots of unity other than $\pm 1$. But the only possibility is that K contains a 3rd, 4th, or 5th root of unity, and so we just pick $b$ to avoid ramification at 2, 3, or 5.

Franz Lemmermeyer · Answer 2 · 2010-02-14 16:26:53Z UTC

8

Dirichlet's unit theorem is a relatively straightforward extension of the well known proof that the Pell equation has a nontrivial solution by a clever use of Dirichlet's box principle. The "machinery" is only needed to control the combinatorial explosion, or, as far as geometry of numbers is concerned, as a natural generalization of the box principle.

As for your second question I first thought that the answer is no. Take for example a biquadratic number field $K = {\mathbb Q}(\sqrt{m},\sqrt{n}\,)$, with $m,n > 0$ chosen in such a way that the unit index (units of K : units from the subfields) is 1. This implies that the only units in K are multiples of those coming from the three subfields. But you still get generators of the field by taking the product of two units coming from different subfields, so your question is still open.

edited Feb 14 2010 at 16:26

answered Feb 14 2010 at 15:39

Franz Lemmermeyer
17.3k●1●38●102

Franz, thanks. I wonder if it's possible to give a good characterization of those fields that do have such a generator. It looks like the most likely exceptions are abelian fields but things may be more subtle than that. – Victor Miller Feb 14 2010 at 15:58

2

I've corrected my initial claims. Hunter has given a counterexample to your question below which can be generalized to most CM fields (totally complex quadratic extensions of totally real fields). For the rest, your question is still open. – Franz Lemmermeyer Feb 14 2010 at 16:30

Franz, so does the argument go something like this: Let $K=\mathbb{Q}(\zeta_n)$ chosen so that the index of of unit group in the real subfield of index 2 in the group of units is 1 (or maybe prime to $2n$). Choose $H$ a proper subgroup of the galois group not containing complex conjugation and index $> 2$, and take the fixed field by $H$. The idea is to kill off the roots of unity, as in Hunter's answer. – Victor Miller Feb 14 2010 at 16:49

9

If K is not a CM field, then K is generated by a unit: K has only finitely many subfields, each of which has lower unit rank than K, so it is possible to choose a unit of K outside these subfield unit groups. If K is a CM field, sometimes K is generated by a unit and sometimes not, but in any case there exists m>0 such that the m-th power of every unit u is in the totally real subfield. – Bjorn Poonen Feb 14 2010 at 18:02

1

The quantity to look at in the CM-case L/K (L totally complex, K totally real, (L:K) = 2) is Hasse's unit index Q=(E_L:W_L E_K), where E_* denotes the unit group and W_* the group of roots of unity in the fields. This index is 1 or 2, and can be computed for quite general families of fields (see Acta Arith. 72 (1995), 347-359 for a collection of known results). If Q = 1, obstructions to the existence of a "generating unit" must come from the roots of unity. If Q = 2, the problem seems to lie deeper, but in any case there is a unit not contained in the real subfield K. – Franz Lemmermeyer Feb 15 2010 at 6:30

show 1 more comment

Two questions about units in Number Fields

Remember to vote up questions/answers you find interesting or helpful (requires 15 reputation points)

2 Answers

You can accept an answer to one of your own questions by clicking the check mark next to it. This awards 15 reputation points to the person who answered and 2 reputation points to you.