On Level One Cuspidal Bianchi Modular Forms

In this paper, we present the outcome of vast computer calculations, locating several of the very rare instances of level one cuspidal Bianchi modular forms that are not lifts of elliptic modular forms.Comment: final versio

On Fermat's equation over some quadratic imaginary number fields

Assuming a deep but standard conjecture in the Langlands programme, we prove Fermat's Last Theorem over $\mathbb Q(i)$. Under the same assumption, we also prove that, for all prime exponents $p \geq 5$, Fermat's equation $a^p+b^p+c^p=0$ does not have non-trivial solutions over $\mathbb Q(\sqrt{-2})$ and $\mathbb Q(\sqrt{-7})$.Comment: The present is a revised version, including suggestions from referees, that was accepted for publication in Research in Number Theory; 16 page

On the cohomology of linear groups over imaginary quadratic fields

Let Gamma be the group GL_N (OO_D), where OO_D is the ring of integers in the imaginary quadratic field with discriminant D<0. In this paper we investigate the cohomology of Gamma for N=3,4 and for a selection of discriminants: D >= -24 when N=3, and D=-3,-4 when N=4. In particular we compute the integral cohomology of Gamma up to p-power torsion for small primes p. Our main tool is the polyhedral reduction theory for Gamma developed by Ash and Koecher. Our results extend work of Staffeldt, who treated the case n=3, D=-4. In a sequel to this paper, we will apply some of these results to the computations with the K-groups K_4 (OO_{D}), when D=-3,-4

On Serre's modularity conjecture and Fermat's equation over quadratic imaginary fields of class number one

In the present article, we extend previous results of the author and we show that when K is any quadratic imaginary field of class number one, Fermat's equation does not have integral coprime solutions such that and is prime. The results are conjectural upon the veracity of a natural generalisation of Serre's modularity conjecture

Twisted limit formula for torsion and cyclic base change

Let $G$ be the group of complex points of a real semi-simple Lie group whose fundamental rank is equal to 1, e.g. G= \SL_2 (\C) \times \SL_2 (\C) or \SL_3 (\C). Then the fundamental rank of $G$ is $2,$ and according to the conjecture made in \cite{BV}, lattices in $G$ should have 'little' --- in the very weak sense of 'subexponential in the co-volume' --- torsion homology. Using base change, we exhibit sequences of lattices where the torsion homology grows exponentially with the \emph{square root} of the volume. This is deduced from a general theorem that compares twisted and untwisted $L^2$-torsions in the general base-change situation. This also makes uses of a precise equivariant 'Cheeger-M\"uller Theorem' proved by the second author \cite{Lip1}.Comment: 23 page