Is the Leopoldt conjecture almost always true? - MathOverflow

Is the Leopoldt conjecture almost always true?

2011-05-28T02:09:10Z

The famous Leopoldt conjecture asserts that for any number field $F$ and any prime $p$, the $p$-adic regulator of $F$ is nonzero. This is known to be equivalent to the vanishing of $H^2(G_{F'/F},\mathbf{Q}_p/\mathbf{Z}_p)$, where $F'$ is the maximal pro-$p$ extension of $F$ unramified outside $p$. When $F/\mathbf{Q}$ is abelian, the conjecture was proven by Brumer.

My question: is there any reasonable sense in which the Leopoldt conjecture is "usually" true - e.g., is it known for any fixed $F$ at almost all primes $p$, or (say) for almost all quartic extensions of $\mathbf{Q}$ with $p$ fixed? A glance through the mathscinet reviews of all the papers with "Leopoldt conjecture" in their title didn't reveal anything, but perhaps this is well-known to experts.

Answer by Kimball for Is the Leopoldt conjecture almost always true?

2011-05-28T04:10:02Z

Leopoldt's conjecture seems to have been proved now by Mihailescu:

http://arxiv.org/abs/0905.1274

In fact before that, I think Fujiwara had done significant work on it (maybe the case of totally real fields?).

Answer by Joël for Is the Leopoldt conjecture almost always true?

2011-06-25T10:07:54Z

I think that not much is known. For example I don't think that we are any closer to prove Leopoldt's conjecture for a given $F$ for infinitely many $p$ than to prove it for all $p$.

Here is a result though: for $K_n=$ cyclotomic fields generated over $\bf Q$ (variante: over a fixed quadrqtic imqginary field; over a fixed totally real field) by the roots of unity of order $p^n$, the "defect" of Leopoldt's conjecture (the dimension of $H^2(K_n'/K_n,{\bf Q}_p)$) stays bounded as $n$ goes to infinity. This is a consequence of the main conjecture known in this case and in the variantes. This is already a very useful result (used for example by Minhyong Kim in his beautifull new proofs of old Diophantine results, such as Siegel theorem for a CM elliptic curve).

Not really in the same spirit, but somehow similar to the brummer proof of the abelian case; it is important to mention Waldschmidt's beautiful result, that the defect in Leopoldt's conjecture is at most half of the degree of $F$.

Since both Brumer's result and Waldschmidt's have proof using fundamentally the theory of transcendence, and for other reasons as well, many people (including myself) think that proving Leopoldt's conjecture will require some transcendence methods (as opposed as methods of algebraic number theory, automorphic forms, etc.). But "generic results" as asked by the question might be more accessible, if by no means simple.

Answer by Preda for Is the Leopoldt conjecture almost always true?

2011-06-29T14:59:46Z

Olivier and all,

If you trust your own minds, you should better try directly and read version 2 of the proof for only CM fields, which I posted in June this years. The rest is hot wind - people may bet, in front of the list of names who failed at Leopoldt you may put the odds for my breakthrough around 1% - but it is only reading which can provide your own judgment of whether this 1 has to be more likely than the complementary 99. I teach the proof in class since 3 weeks and it works quite fluidly and the students can grab the construction very well - useless to say, it is enriched by many details, since it is a 3-d year course (guess something like first graduate year). I gave up the construction of techniques for non CM fields, the Iwasawa skew symmetric pairing, and reduced to the skeletton of the principal ideas, exactly in order to respond to the loud whispers about my expressivity.

As for the Cambridge seminar mentioned, it was a great experience - but it happened during a week loaded with important other seminars, and in spite of the particular attention offered, we did not have more than 3 or 4 meetings of two hours, this was certainly not enough for completing a proof with all the details, just the time for gathering some important questions and find out on what particular issue people would like to know more. This is taken into account in the present version.

It is also true that Minhyong Kim, this friendly and enthusiastic fellow, asked for my allowance to put the draft on the blog, exactly in the expectation that more students and young researchers would just try and bite at it, and raise questions, which were very welcome. The expected impact did not happen. Therefore I friendly invite you to simply read. Anyone having concrete questions is gladly invited to write me a mail, if I understand the question his chances are one in a thousand that I will not respond.

Sorry if I intruded your discussion

Preda Mihailescu