Eisenstein series for G₂  and the symmetric cube Bloch--Kato conjecture
Creators: Mundy, Samuel Raymond
Publisher: name: Columbia University
Year: 2021
Subjects: subject: Mathematics
subject: FOS: Mathematics
subject: FOS: Mathematics
subject: Eisenstein series
subject: Automorphic functions

description: The purpose of this thesis is to construct nontrivial elements in the Bloch--Kato Selmer group of the symmetric cube of the Galois representation attached to a cuspidal holomorphic eigenform 𝐹 of level 1. The existence of such elements is predicted by the Bloch--Kato conjecture. This construction is carried out under certain standard conjectures related to Langlands functoriality. The broad method used to construct these elements is the one pioneered by Skinner and Urban in [SU06a] and [SU06b]. The construction has three steps, corresponding to the three chapters of this thesis. The first step is to use parabolic induction to construct a functorial lift of 𝐹 to an automorphic representation π of the exceptional group G₂ and then locate every instance of this functorial lift in the cohomology of G₂. In Eisenstein cohomology, this is done using the decomposition of Franke--Schwermer [FS98]. In cuspidal cohomology, this is done assuming Arthur's conjectures in order to classify certain CAP representations of G₂ which are nearly equivalent to π, and also using the work of Adams--Johnson [AJ87] to describe the Archimedean components of these CAP representations. This step works for 𝐹 of any level, even weight 𝑘 ≥ 4, and trivial nebentypus, as long as the symmetric cube 𝐿-function of 𝐹 vanishes at its central value. This last hypothesis is necessary because only then will the Bloch--Kato conjecture predict the existence of nontrivial elements in the symmetric cube Bloch--Kato Selmer group. Here this hypothesis is used in the case of Eisenstein cohomology to show the holomorphicity of certain Eisenstein series via the Langlands--Shahidi method, and in the case of cuspidal cohomology it is used to ensure that relevant discrete representations classified by Arthur's conjecture are cuspidal and not residual. The second step is to use the knowledge obtained in the first step to 𝓅-adically deform a certain critical 𝓅-stabilization 𝜎π of π in a generically cuspidal family of automorphic representations of G₂. This is done using the machinery of Urban's eigenvariety [Urb11]. This machinery operates on the multiplicities of automorphic representations in certain cohomology groups; in particular, it can relate the location of π in cohomology to the location of 𝜎π in an overconvergent analogue of cohomology and, under favorable circumstances, use this information to 𝓅-adically deform 𝜎π in a generically cuspidal family. We show that these circumstances are indeed favorable when the sign of the symmetric functional equation for 𝐹 is -1 either under certain conditions on the slope of 𝜎π, or in general when 𝐹 has level 1. The third and final step is to, under the assumption of a global Langlands correspondence for cohomological automorphic representations of G₂, carry over to the Galois side the generically cuspidal family of automorphic representations obtained in the second step to obtain a family of Galois representations which factors through G₂ and which specializes to the Galois representation attached to π. We then show this family is generically irreducible and make a Ribet-style construction of a particular lattice in this family. Specializing this lattice at the point corresponding to π gives a three step reducible Galois representation into GL₇, which we show must factor through, not only G₂, but a certain parabolic subgroup of G₂. Using this, we are able to construct the desired element of the symmetric cube Bloch--Kato Selmer group as an extension appearing in this reducible representation. The fact that this representation factors through the aforementioned parabolic subgroup of G₂ puts restrictions on the extension we obtain and guarantees that it lands in the symmetric cube Selmer group and not the Selmer group of 𝐹 itself. This step uses that 𝐹 is level 1 to control ramification at places different from 𝓅, and to ensure that 𝐹 is not CM so as to guarantee that the Galois representation attached to π has three irreducible pieces instead of four.