The generating series of a number of different objects studied in arithmetic statistics can be built out of Euler products. Euler products often have very nice analytic properties, and by constructing a meromorphic continuation one can use complex analytic techniques, including Tauberian theorems to prove asymptotic counting theorems for these objects. One standard technique for producing a meromorphic continuation is to factor out copies of the Riemann zeta function, for which a meromorphic continuation is already known.

This paper is an exposition of the ``Factorization Method" for meromorphic continuation. We provide the following three resources with an eye towards research in arithmetic statistics: (1) an introduction to this technique targeted at new researchers, (2) exposition of existing works, with self-contained proofs, that give a continuation of Euler products with constant or Frobenain coefficients to the right halfplane Re(s)>0 (away from an isolated set of singularities), and (3) explicit statements on the locations and orders of all singularities for these Euler products.

We prove significant power savings for the error term when counting abelian extensions of number fields (as well as the twisted version of these results for nontrivial Galois modules). In some cases over Q, these results reveal lower order terms following the same structure as the main term that were not previously known. Assuming the generalized Lindel\"of hypothesis for Hecke L-functions, we prove square root power savings for the error compared to the order of the main term.

We prove necessary and sufficient conditions for a finite group G with an ordering of G-extensions to satisfy the following property: for every positive density set of places A of a number field K and every splitting type given by a conjugacy class c in G, 0% of G-extensions avoid this splitting type for each p in A.

We define a group with local data over a number field K as a group G together with homomorphisms from decomposition groups of K_p to G. Such groups resemble Galois groups, just without global information. Motivated by the use of random groups in the study of class group statistics, we use the tools given by Sawin-Wood to construct a random group with local data over K as a model for the absolute Galois group for which representatives of Frobenius are distributed Haar randomly as suggested by Chebotarev density. We utilize Law of Large Numbers results for categories proven by the author to show that this is a random group version of the Malle-Bhargava principle. In particular, it satisfies number field counting conjectures such as Malle's Conjecture under certain notions of probabilistic convergence including convergence in expectation, convergence in probability, and almost sure convergence. These results produce new heuristic justifications for number field counting conjectures, and begin bridging the theoretical gap between heuristics for number field counting and class group statistics.

In arithmetic statistics and analytic number theory, the asymptotic growth rate of counting functions giving the number of objects with order below X is studied as X tends towards infinity. We define general counting functions which count epimorphisms out of an object on a category under some ordering. Given a probability measure μ on the isomorphism classes of the category with sufficient respect for a product structure, we prove a version of the Law of Large Numbers to give the asymptotic growth rate as n tends towards infinity of such functions with probability 1 in terms of the finite moments of μ and the ordering. Such counting functions are motivated by work in arithmetic statistics, including number field counting as in Malle's conjecture and point counting as in the Batyrev-Manin conjecture. Recent work of Sawin--Wood gives sufficient conditions to construct such a measure μ from a well-behaved sequence of finite moments in very broad contexts, and we prove our results in this broad context with the added assumption that a product structure in the category is respected. These results allow us to formalize vast heuristic predictions about counting functions in general settings.

Malle proposed a conjecture for counting the number of G-extensions L/K with discriminant bounded above by X, denoted N(K,G;X), where G is a fixed transitive subgroup G⊂Sn and X tends towards infinity. We introduce a refinement of Malle’s conjecture, if G is a group with a nontrivial Galois action then we consider the set of crossed homomorphisms in Z^1(K,G) (or equivalently 1-coclasses in H^1(K,G)) with bounded discriminant. This has a natural interpretation given by counting G-extensions F/L for some fixed L and prescribed extension class F/L/K. If T is an abelian group with any Galois action, we compute the asymptotic main term of this refined counting function for Z^1(K,T) (and equivalently for H^1(K,G)) and show that it is a natural generalization of Malle’s conjecture. The proof technique is in essence an application of a theorem of Wiles on generalized Selmer groups, and additionally gives the asymptotic growth rate when restricted to certain local behaviors. As a consequence, whenever the inverse Galois problem is solved for G⊂Sn over K and G has an abelian normal subgroup T≤G we prove a nontrivial lower bound for N(K,G;X) given by a nonzero power of X times a power of logX. For many groups, including many solvable groups, these are the first known nontrivial lower bounds. These bounds prove Malle’s predicted lower bounds for a large family of groups, and for an infinite subfamily they generalize Klüners’ counter example to Malle’s conjecture and verify the corrected lower bounds predicted by Turkelli. 

We utilize harmonic analytic tools to count the number of elements of the Galois cohomology group f in H^1(K,T) with discriminant-like invariant inv(f)≤X as X tends to infinity. Specifically, Poisson summation produces a canonical decomposition for the corresponding generating series as a sum of Euler products for a very general counting problem. This type of decomposition is exactly what is needed to compute asymptotic growth rates using a Tauberian theorem. These new techniques allow for the removal of certain obstructions to known results and answer some outstanding questions on the generalized version of Malle's conjecture for the first Galois cohomology group.

In our original paper, there are special cases in which the main theorem could not hold for reasons related to the Grunwald--Wang theorem. We correct the statement and its proof in the corrigendum, and we include a short discussion of the added hypothesis of ``viability'' needed to make our theorem true.

Note: The arxiv version has been update to include the content of both the original paper and the corrigendum as a cohesive unit.

Cohen and Lenstra detailed a heuristic for the distribution of odd p-class groups for imaginary quadratic fields. One such formulation of this distribution is that the expected number of surjections from the class group of an imaginary quadratic field k to a fixed odd abelian group is 1. Class field theory tells us that the class group is also the Galois group of the Hilbert class field, the maximal unramified abelian extension of k, so we could equivalently say the expected number of unramified G-extensions of k is 1/#Aut(G) for a fixed abelian group G. We generalize this to asking for the expected number of unramified G-extensions Galois over k for a fixed finite group G, with no restrictions placed on G. We review cases where the answer is known or conjectured by Boston-Wood, Boston-Bush-Hajir, and Bhargava, then answer this question in several new cases. In particular, we show when the expected number is zero and give a nontrivial family of groups realizing this. Additionally, we prove the expected number for the quaternion group Q8 and dihedral group D4 of order 8 is infinite. Lastly, we discuss the special case of groups generated by elements of order 2 and give an argument for an infinite expected number based on Malle’s conjecture. 

We compute all the moments of a normalization of the function which counts unramified H8-extensions of quadratic fields, where H8 is the quaternion group of order 8, and show that the values of this function determine a constant distribution. Furthermore we propose a similar modification to the non-abelian Cohen-Lenstra heuristics for unramified G-extensions of quadratic fields for G in a large class of 2-groups, which we conjecture will give finite moments which determine a distribution. Our method additionally can be used to determine the asymptotics of the unnormalized counting function, which we also do for unramified H8-extensions. 

For a fixed finite solvable group G and number field K, we prove an upper bound for the number of G-extensions L/K with restricted local behavior (at infinitely many places) and inv(L/K)<X for a general invariant “inv”. When the invariant is given by the discriminant for a transitive embedding of a nilpotent group G⊂Sn, this realizes the upper bound given in the weak form of Malle’s conjecture. For other solvable groups, the upper bound depends on the size of torsion in the class group of number fields with fixed degree. In particular, the bounds we prove realize the upper bound given in the weak form of Malle’s conjecture for the transitive embedding of a solvable group G⊂Sn if we assume that for each finite abelian group A the average size of class group torsion |Hom(Cl(L),A)| is smaller than X^ϵ as L/K varies over certain families of extensions with inv(L/K)<X. 

We study solutions to the Brauer embedding problem with restricted ramification. Suppose G and A are a abelian groups, E is a central extension of G by A, and f:Gal(\overline{\Q}/\Q)→G a continuous homomorphism. We determine conditions on the discriminant of f that are equivalent to the existence of an unramified lift f˜:Gal(\overline{\Q}/\Q)→E of f. As a consequence of this result, we use conditions on the discriminant of K for K/\Q abelian to classify and count unramified nonabelian extensions L/K normal over \Q where the (nontrivial) commutator subgroup of Gal(L/\Q) is contained in its center. This generalizes a result due to Lemmermeyer, which states that a quadratic field \Q(\sqrt{d}) has an unramified extension normal over Q with Galois group H8 the quaternion group if and only if the discriminant factors d=d1 d2 d3 as a product of three coprime discriminants, at most one of which is negative, satisfying the condition (di dj / pk)=1 on Legendre symbols for each {i,j,k}={1,2,3} and pk a prime dividing dk. 

My PhD thesis is based on the work in the four preprints I completed during my time in graduate school from 2013-2018, namely "Cohen-Lenstra Moments for Some Nonabelian Groups", "The distribution of $H_8$-extensions of quadratc fields", "Certain Unramified Metabelian Extensions Using Lemmermeyer Factorizations", and "The Weak Form of Malle's Conjecture and Solvable Groups". I reformatted the material from these preprints to have consistent notation, narrative, and a more detailed introduction to create my thesis.

Be warned: some of these papers have been peer reviewed during the publication process after my graduation, and the final results involve both minor and major improvements and corrections. While the narrative and end results are essentially the same as my thesis, be warned that some of the proof details in my thesis may no longer resemble the details in my published work. You should refer to the published papers for the most up to date versions of the research in my thesis.

PhD advisor: Nigel Boston

Dissertation: Galois Cohomology and Number Field Counting 

Defense: Slides