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authorSebastiano Tronto <sebastiano@tronto.net>2023-05-06 17:43:11 +0200
committerSebastiano Tronto <sebastiano@tronto.net>2023-05-06 17:43:11 +0200
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parent11f155d5a60715508a3fe2069f723f354c65bc52 (diff)
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@@ -5,10 +5,11 @@ After obtaining a bachelor degree in Trento in 2016 I have completed the
5special focus on number theory and algebraic geometry. I then moved on to 5special focus on number theory and algebraic geometry. I then moved on to
6obtain a joint PhD at the universities of Luxembourg and Leiden, under 6obtain a joint PhD at the universities of Luxembourg and Leiden, under
7the supervision of [Antonella Perucca](http://antonellaperucca.net) and 7the supervision of [Antonella Perucca](http://antonellaperucca.net) and
8[Peter Bruin](https://www.math.leidenuniv.nl/~pbruin). I have collected 8[Peter Bruin](https://www.math.leidenuniv.nl/~pbruin).
9in this page the list of papers I puplished during my PhD, as well as 9
10slides or notes for all the talks I have given and my theses (Bachelor, 10I have collected in this page the list of papers I puplished during my PhD,
11Master, PhD). 11as well as theses (Bachelor, Master, PhD). See [my talks page](../talks)
12for a list of slides and notes of my talks.
12 13
13## Theses 14## Theses
14 15
@@ -69,100 +70,3 @@ Master, PhD).
69* Perucca, Antonella; Sgobba, Pietro; Tronto, Sebastiano. 70* Perucca, Antonella; Sgobba, Pietro; Tronto, Sebastiano.
70 *Explicit Kummer theory for the rational numbers.* 71 *Explicit Kummer theory for the rational numbers.*
71 International Journal of Number Theory (2020). 72 International Journal of Number Theory (2020).
72
73## Talks
74
75* *Kummer theory for elliptic curves*.
76 Short talk for Tarrach Prize 2023, March 2023.
77 [Slides: [pdf, 288Kb](kummer-tarrach.pdf)]
78
79* *Kummer theory for commutative algebraic groups*.
80 Seminar at the university of Leiden, September 2022.
81 [Slides: [pdf, 1.2Mb](slides-kummer-final.pdf)]
82
83* *My journey in Kummer theory*.
84 Leiden-Luxembourg Number Theory PhD Days 2022, July 5-6.
85 [Slides: [pdf, 521Kb](slides-journey-kummer.pdf)]
86
87* *A tour of group theory with our companion cube*.
88 University of Luxembourg PhD seminar, May 2022.
89 [Slides: [pdf, 1.1Mb](group-cube.pdf)]
90
91* *Kummer theory via (J,T)-extensions*.
92 University of Leiden algebra seminar, May 2022;
93 similar to *A category of division modules*, see below.
94
95* *Kummer theory for algebraic groups*.
96 Nederlands Matematisch Congres, April 2022;
97 repeated at the DIAMANT symposium, April 2022.
98 [Notes (NMC): [pdf, 215Kb](slides-kummer-kwg.pdf);
99 Script (NMC): [txt, 9Kb](script-kummer-kwg.txt);
100 Notes (DIAMANT, longer version): [pdf, 223Kb](slides-kummer-diamant.pdf)]
101
102* *Division in modules*.
103 University of Leiden algebra seminar, April 2022;
104 similar to *A generalization of injective modules* and
105 *Division in modules and Kummer theory*, see below.
106 [Notes: [pdf, 245Kb](division-leiden.pdf)]
107
108* *Division in modules and Kummer theory*.
109 Invited talk at the University of Groningen, February 2022.
110 [Slides: [pdf, 249Kb](division-groningen.pdf)]
111
112* *A category of division modules*.
113 University of Luxembourg Number Theory seminar, November 2021.
114 [Notes: [pdf, 270Kb](notes-division-modules.pdf)]
115
116* *Introduction to étale cohomology*.
117 University of Luxembourg seminar series on perverse sheaves, October 2021.
118 [Notes: [pdf, 237Kb](notes-etale.pdf)]
119
120* *A generalization of injective modules*.
121 University of Luxembourg Number Theory seminar, October 2021.
122 [Notes: [pdf, 259Kb](notes-injectivity.pdf)]
123
124* *Integer factorization and elliptic curves*.
125 University of Leiden Bachelor seminar (guest talk), April 2021.
126 [Slides: [pdf, 393Kb](slides-ecm.pdf)]
127
128* *Group cohomology and elliptic curves*.
129 Invited talk at Number Theory Online conference, February 2021.
130 [Slides: [pdf, 210Kb](slides-groupcohomec.pdf);
131 Video: [link](https://vimeo.com/526814236/1a640285c0?embedded=true&source=video_title&owner=47245911)]
132
133* *Kummer theory for algebraic groups*.
134 Invited talk at the University of Bristol (online), October 2020.
135 [Slides: [pdf, 239Kb](slides-tronto-bristol.pdf)]
136
137* *Algebraic groups and field extensions*.
138 University of Luxembourg PhD seminar (online), April 2020.
139 [Slides: [pdf, 280Kb](slides-alggroupsfieldext.pdf)]
140
141* *Field extensions and elliptic curves*.
142 University of Luxembourg PhD Day, October 2019.
143 [Slides: [pdf, 6.0Mb](slides-fieldextec.pdf)]
144
145* *Kummer theory for elliptic curves*.
146 University of Leiden algebra seminar, September 2019;
147 repeated with slides at the DIAMANT symposium, November 2019 and
148 at Seminari de Teoria de Nombres de Barcelona, February 2020.
149 [Notes: [pdf, 181Kb](notes-kummerec.pdf);
150 Slides DIAMANT: [pdf, 271Kb](slides-kummec-diamant.pdf);
151 Slides Barcelona: [pdf, 290Kb](slides-kummec-barcelona.pdf)].
152
153* *Divisibility of points in algebraic groups*.
154 Invited talk at the University of the Basque Country, May 2019.
155 [Notes: [pdf, 231Kb](notes-bilbao.pdf)]
156
157* *Kummer theory for number fields*.
158 Fifth symposium of the Roman Number Theory association, April 2019;
159 repeated at the Algant Alumni symposium in Benasque, May 2019.
160 [Slides: [pdf, 291Kb](slides-kummerdegrees.pdf)]
161
162* *The local-global principle*.
163 University of Luxembourg interdisciplinary seminar, March 2019.
164 [Slides: [pdf, 598Kb](slides-local-global.pdf)]
165
166* *Local-global principle for torsion*.
167 University of Luxembourg Number Theory seminar, November 2018.
168 [Notes: [pdf, 208Kb](notes-katz.pdf)]
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1--- Kummer theory ---
20:50
3
4Kummer theory is the study of field extensions generated by the n-th roots
5of elements of a base field, such as the rational numbers. Taking all
6n-th roots ensures that such an extension is Galois and that it contains
7the cyclotomic field: indeed, one can write the n-th roots of unity as
8ratios of different n-th roots of the same element.
9
10To study these extensions it is convenient to start not just with a
11set of elements, but with a multiplicative subgroup of the base field.
12For example, one can take the group generated by one element. This does
13not change anything on the field-theoretic side: the extensions we are
14considering do not change. But it makes the "purely algebraic" side of
15things more convenient: the group sqrt[n]{A} now is group which contains
16A and the roots of unity.
17
18
19--- Kummer theory for algebraic groups ---
201:00
21
22"Kummer theory for algebraic groups" is a similar, more general
23problem. If we take G to be a commutative algebraic group over a number
24field K, we can take a subgroup of the K-rational points of G and you
25consider the n-division points of this group, which is an analogue of
26the group of n-th roots in the previous case.
27
28If you add the coordinates of these points to your base field you obtain
29what a field extension which has properties remarkably similar to the
30classical Kummer extensions: it is Galois over K and it contains the
31n-torsion field of G, an analogue of the cyclotomic field generated by
32the torsion points of G.
33
34This is a generalization of the classical case, because if you take G
35to be the multiplicative group you obtain exactly that case.
36
37These field extensions are the kind of objects that I am studying.
38
39
40--- Results for elliptic curves ---
411:50
42
43So, what do we want to know about these kind of field extensions? One of
44the things we care about is estimating, or computing, their degrees.
45This is because they have applications in other areas of number theory:
46when studying problems related to Artin's primitive root conjecture it
47can happen that the density of certain set of primes can be expressed
48in terms of the degrees of Kummer extensions.
49
50The degree of a Kummer extension over the torsion field is always
51between a certain power of n and the same power of n times a constant.
52This power of n is for example 2 in the case of elliptic curves and
53groups of points generated by one non-torsion element. So they cannot
54be much smaller than the maximum.
55
56In recent years there has been effort in making these results effective.
57In a recent work with Lombardo we were able to quantify this constant, or
58a possible value for it, in terms of computable properties of the curve
59and of the chosen point, for curves without complex multiplication. In
60particular, one of these properties are the p-adic Galois representation
61associated with the curve. Over Q we even have an explicit and uniform
62estimate for such a constant.
63
64In our work we had problems when the curve had non-trivial endomorphisms
65defined over the base field, so CM curves. However Abtien Javan Peykar, a
66student of Lenstra, managed to get similar results for CM curves only. So,
67how did he manage?
68
69
70--- Endomorphism rings ---
710:55
72
73The problem Lombardo and I had was caused by considering the A and its
74division groups as abelian groups. Instead, Javan Peykar decided to
75take them as modules over the endomorphism ring of the curve, an order
76in a quadratic imaginary field - and even with some extra technical
77limitations, such as considering only maximal orders.
78
79So my idea was: if we can do the same over a *general* ring, regardless
80of it being Z or an order in a number field or anything else, maybe we
81can build a general framework to study these division groups, or rather
82division modules, and then apply all of this to do Kummer theory over
83other classes of algebraic groups.
84
85So this is what I did.
86
87
88--- Division modules ---
891:00
90
91The first part is understanding what "division in modules is", starting
92from the "denominator": what do we divide by?
93
94As is often the case in commutative algebra, it is convenient to use,
95instead of the elements of the ring, ideals of the ring. So for M
96contained in N we define the I-division module of M inside N to be the
97set of elements of N that multiplied by I end up inside M. This is a
98classical definition that is found in some commutative algebra books.
99
100We also want to consider infinite unions of such division modules. For
101example we might want to work with the set of all division points of
102our subgroup of rational points. In our first example, all n-th roots
103of a certain number.
104
105To do this, we introduce the concept of "ideal filter", which like a
106fiter in set theory but for ideals.
107
108
109--- Ideal filters ---
1100:20
111
112In practice I always want to divide by one of these two families of
113ideals: either the one generated by all positive integers or the one
114generated by powers of a given prime.
115
116These are the main example of ideal filters that we should keep in mind,
117but I will develop my theory in general.
118
119
120--- J-injectivity ---
1211:25
122
123The set of all division points is a divisible abelian group. But over
124a general ring this divisibility property can be awkward to work with,
125and we prefer to use injectivity, which is equivalent to divisibility
126over the ring Z.
127
128A module is called injective when maps to it can be lifted along
129injective morphisms. We call it instead J-injective when maps to it can
130be lifted along certain injective morphisms, namely those such that the
131codomain coincides with the module of J-division points of the image.
132This definition captures the concept of "dividing only by J".
133
134This a nice and simple generalization of a classical concepts, but it has
135some noteworthy properties. First of all it is a true generalization:
136taking J to be the set of all right ideals of R it becomes equivalent
137to injectivity. And for example one can use it to extend the definition
138of p-divisible abelian group: over Z p-divisibility is equivalent to
139p^\infty-injectivity, where p^\infty is the ideal filter I introduced
140in the previous slide.
141
142One might say that this definition highlights the connection between
143injectivity and divisibility better than the classical one does.
144
145
146--- (J,T)-extensions ---
1471:20
148
149The last ingredient to complete our algebraic theory is the torsion. We
150are building all this theory of division modules abstractly, in a way
151independent of the agebraic group G that we started with. But when we do
152this and we consider division modules, there is no way for this objects
153to know that they are supposed to live in some elliptic curve rather than
154in the multiplicative group or in some higher-dimensional abelian variety.
155
156We need some extra structure. We need to fix a torsion and J-injective
157module T that plays the role of the torsion subgroup of G.
158
159Then we consider only those extensions of a base module M that consist
160of division points and whose torsion embeds into T.
161
162These objects form a category with many nice properties, that strongly
163resembles the category of field extensions of a fixed field.
164
165We also have an analogue of an algebraic closure, that plays the role
166of the "set of all division points" that we have mentioned. This can be
167constructed as a "J-hull", which is the analogue of the injective hull,
168or injective envelope, for our generalization of injectivity.
169
170With this category we can establish many properties of division modules,
171and study their automorphisms.
172
173
174--- Galois representations ---
1750:55
176
177And finally, how do I use this whole theory to study my number theoretical
178problems? I can consider the Galois group of my Kummer extension,
179say the one generated by all division points, and it embeds into the
180automorphism group of this maximal (J,T)-extension.
181
182This automorphism group fits into a short exact sequence.
183
184Then the standard short exact sequence of Galois theory embeds into this
185and we obtain this commutative diagram of groups, with exact rows. This
186sequence is the main tool to study Kummer theory for algebraic groups.
187
188Finding an explicit lower bound for the degrees I talked about amounts
189to proving an explicit open image theorem for this "representation"
190on the left-hand side.
191
192In short, this diagram is the key to derive number-theoretic results
193from certain key properties of the group.
194
195
196--- New results ---
1970:50
198
199So, what kind of new results were we able to obtain with this technical
200tools in our hands?
201
202First of all, it was easy to unify the CM and non-CM cases and
203show that one does not need to separate the two cases, except for
204studying some specific properties of the curves related to their Galois
205representations. Once you have have them, you can plug in any elliptic
206curve with any endomorphism ring into our general framework and you
207obtain the (already known) results. This also completes the CM case,
208that had some missing pieces due to technical difficulties.
209
210More importantly, in my opinion, we have now a better understanding of
211these objects.
212
213You see, when studying a problem cases by case is like you are trying
214to find your way in a forest step by step. With this general framework
215we have a way-better overview of the landscape we are moving in.
216
217Lastly, the generality of this theory allows one to obtain some results
218for higher-dimensional abelian varieties. This is work in progress,
219but we already have results for some classes of varieties. There some
220technical things to work out related to understanding the torsion subgroup
221as a module over the endomorphism ring, but I am optimistic that we will
222work this out.
223
224
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