Leopoldt conjecture
Let
be a totally real algebraic number field (cf. also Field; Algebraic number) and let p
be a prime number. Let {\sigma _ {1} \dots \sigma _ {r _ {1} } } : F \rightarrow {\mathbf C _ {p} }
denote the distinct embeddings of F
into the completion \mathbf C _ {p}
of the algebraic closure of \mathbf Q _ {p} .
By the Dirichlet unit theorem (cf. also Dirichlet theorem), the unit group U _ {F}
of F
has rank r = r _ {1} - 1 .
Let \epsilon _ {1} \dots \epsilon _ {r}
be a \mathbf Z -
basis of U _ {F} .
In [a5], H.-W. Leopoldt defined the p -
adic regulator R _ {p} ( F )
as the p -
adic analogue of the Dirichlet regulator:
R _ {p} ( F ) = \pm { \mathop{\rm det} } \left ( { \mathop{\rm log} } _ {p} ( \sigma _ {i} ( \epsilon _ {j} ) ) _ {1 \leq i,j \leq r } \right ) ,
where { { \mathop{\rm log} } _ {p} } : {U _ {F} } \rightarrow {\mathbf C _ {p} } denotes the p - adic logarithm.
Leopoldt's conjecture is: R _ {p} ( F ) \neq 0 .
The definition of R _ {p} ( F ) ( and therefore also the conjecture) extends to arbitrary number fields (cf. [a7]) and is nowadays considered in this generality. A. Brumer used transcendental methods developed by A. Baker to prove Leopoldt's conjecture for fields F that are Abelian over \mathbf Q or over an imaginary quadratic field [a2]. For specific non-Abelian fields the conjecture has also been verified (cf., e.g., [a1]), but in general it is still (1996) open.
For a totally real field F , Leopoldt's conjecture is equivalent to the non-vanishing of the p - adic \zeta - function \zeta _ {F,p } ( s ) at s = 1 ( cf. [a5], [a3]).
For a prime v in F , let U _ {v} denote the group of units of the local field F _ {v} . There is a canonical mapping
{f _ {p} } : {U _ {F} \otimes \mathbf Z _ {p} } \rightarrow {\prod _ { {v \mid p } } U _ {v} }
and the Leopoldt defect \delta _ {F} is defined as the \mathbf Z _ {p} - rank of { \mathop{\rm ker} } f _ {p} . Class field theory yields the following equivalent formulation of the Leopoldt conjecture (cf. [a7]): Leopoldt's conjecture holds if and only if \delta _ {F} = 0 .
Relation to Iwasawa theory.
An extension F _ \infty /F of a number field F is called a \mathbf Z _ {p} - extension if it is a Galois extension and { \mathop{\rm Gal} } ( F _ \infty /F ) \cong \mathbf Z _ {p} . The number of independent \mathbf Z _ {p} - extensions of F is related via class field theory to the \mathbf Z _ {p} - rank of { \mathop{\rm coker} } f _ {p} and is equal to 1 + r _ {2} ( F ) + \delta _ {F} ( cf. [a4]), where r _ {2} ( F ) is the number of pairs of complex-conjugate embeddings of F .
For n \geq 0 , let F _ {n} denote the unique subfield of F _ \infty /F of degree p ^ {n} over F and let \delta _ {n} denote the Leopoldt defect of F _ {n} . The \mathbf Z _ {p} - extension F _ \infty /F satisfies the weak Leopoldt conjecture if the defects \delta _ {n} are bounded independent of n . It is known (cf. [a4]) that the weak Leopoldt conjecture holds for the so-called cyclotomic \mathbf Z _ {p} - extension of F , i.e. for the unique \mathbf Z _ {p} - extension contained in F ( \mu _ {p ^ \infty } ) .
Relation to Galois cohomology.
Let G _ {p} ( F ) denote the Galois group of the maximal pro- p - extension of F , which is unramified outside p . Leopoldt's conjecture is equivalent to the vanishing of the Galois cohomology group H ^ {2} ( G _ {p} ( F ) , \mathbf Q _ {p} / \mathbf Z _ {p} ) [a6]. More generally, it is conjectured that
H ^ {2} ( G _ {p} ( F ) , \mathbf Q _ {p} / \mathbf Z _ {p} ( i ) ) = 0
for all i \neq 1 [a6]. This is known to be true for i \geq 2 as a consequence of a profound result of A. Borel in algebraic K - theory.
References
[a1] | F. Bertrandias, J.-J. Payan, "\Gamma-extensions et invariants cyclotomiques" Ann. Sci. Ecole Norm. Sup. (4) , 5 (1972) pp. 517–543 MR0480419 MR0337882 Zbl 0246.12005 Zbl 0246.12004 |
[a2] | A. Brumer, "On the units of algebraic number fields" Mathematica , 14 (1967) pp. 121–124 MR0220694 Zbl 0171.01105 |
[a3] | P. Colmez, "Résidu en s=1 des fonctions zêta p-adiques" Invent. Math. , 91 (1988) pp. 371–389 |
[a4] | K. Iwasawa, "On \ZZ_\ell-extensions of algebraic number fields" Ann. of Math. , 98 (1973) pp. 246–326 MR349627 |
[a5] | H.-W. Leopoldt, "Zur Arithmetik in abelschen Zahlkörpern" J. Reine Angew. Math. , 209 (1962) pp. 54–71 MR0139602 Zbl 0204.07101 |
[a6] | P. Schneider, "Über gewisse Galoiscohomologiegruppen" Math. Z. , 168 (1979) pp. 181–205 MR0544704 Zbl 0421.12024 |
[a7] | L.C. Washington, "Introduction to cyclotomic fields" , Springer (1982) MR0718674 Zbl 0484.12001 |
Leopoldt conjecture. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Leopoldt_conjecture&oldid=53483