Companion forms and weight one forms

In this paper we prove the following theorem. Let L/\Q_p be a finite extension with ring of integers O_L and maximal ideal lambda. Theorem 1. Suppose that p >= 5. Suppose also that \rho:G_\Q -> GL_2(O_L) is a continuous representation satisfying the following conditions. 1. \rho ramifies at only finitely many primes. 2. \rho mod \lambda is modular and absolutely irreducible. 3. \rho is unramified at p and \rho(Frob_p) has eigenvalues \alpha and \beta with distinct reductions modulo \lambda. Then there exists a classical weight one eigenform f = \sum_{n=1}^\infty a_m(f) q^m and an embedding of \Q(a_m(f)) into L such that for almost all primes q, a_q(f)=tr(\rho(\Frob_q)). In particular \rho has finite image and for any embedding i of L in \C, the Artin L-function L(i o \rho, s) is entire.Comment: 15 pages, published version, abstract added in migratio

Playing simple loony dots and boxes endgames optimally

We explain a highly efficient algorithm for playing the simplest type of dots and boxes endgame optimally (by which we mean "in such a way so as to maximise the number of boxes that you take"). The algorithm is sufficiently simple that it can be learnt and used in over-the-board games by humans. The types of endgames we solve come up commonly in practice in well-played games on a 5x5 board and were in fact developed by the authors in order to improve their over-the-board play.Comment: 20 pages; minor revisions made after referee's report. To be published in "Integers

Explicit reduction modulo p of certain 2-dimensional crystalline representations, II

We complete the calculations begun in [BG09], using the p-adic local Langlands correspondence for GL2(Q_p) to give a complete description of the reduction modulo p of the 2-dimensional crystalline representations of G_{Q_p} of slope less than 1, when p > 2.Comment: 10 pages. Correcting a minor typ

Stably uniform affinoids are sheafy

We develop some of the foundations of affinoid pre-adic spaces without Noetherian or finiteness hypotheses. We give some explicit examples of non-adic affinoid pre-adic spaces (including a locally perfectoid one). On the positive side, we also show that if every affinoid subspace of an affinoid pre-adic space is uniform, then the structure presheaf is a sheaf; note in particular that we assume no finiteness hypotheses on our rings here. One can use our result to give a new proof that the spectrum of a perfectoid algebra is an adic space.Comment: Version 2 of the manuscript -- the arguments are now presented for general f-adic rings with a topologically nilpotent unit (the original proofs still go through in this generality

The 2-adic Eigencurve is Proper

For p=2 and tame level N=1 we prove that the map from the (Coleman-Mazur) Eigencurve to weight space satisfies the valuative criterion of properness. More informally, we show that the Eigencurve has no "holes"; given a punctured disc of finite slope overconvergent eigenforms over weight space, the center can be "filled in" with a finite slope overconvergent eigenform

Explicit reduction modulo pp of certain crystalline representations

We use the p-adic local Langlands correspondence for GL_2(Q_p) to explicitly compute the reduction modulo p of crystalline representations of small slope, and give applications to modular forms.Comment: 10 pages, appeared in IMRN 2009, no. 12. This version does not incorporate any minor changes (e.g. typographical changes) made in proo