Learning Theory from First Principles

Francis Bach

Mastere MASH 2022/2023

The class will be taught in French or English, depending on attendance (all slides and class notes are in English).

Summary

The goal of this class is to present old and recent results in learning theory, for the most widely-used learning architectures. This class is geared towards theory-oriented students as well as students who want to acquire a basic mathematical understanding of algorithms used throughout the masters program.

A particular effort will be made to prove many results from first principles, while keeping the exposition as simple as possible. This will naturally lead to a choice of key results that show-case in simple but relevant instances the important concepts in learning theory. Some general results will also be presented without proofs.

The class will be organized in eight three-hour sessions, each with a precise topic. See tentative schedule below. Credit: 5 ECTS.

Prerequisites: We will prove results in class so a good knowledge of undergraduate mathematics is important, as well as basic notions in probability. Having followed an introductory class on machine learning is beneficial.

Dates

All classes will be "in real life" at ENS (29, rue d'Ulm), on Friday between 9am and 12pm, in the room Paul Langevin (1st floor), except on December 9.

The class will follow the book in preparation (draft available here, since it will be updated frequently, please get the latest version).

Each student will benefit more from the class is the corresponding sections are read before class.

Date	Topics	Book chapters Figures to reproduce
October 7	Learning with infinite data (population setting) -Decision theory (loss, risk, optimal predictors) -Decomposition of excess risk into approximation and estimation errors -No free lunch theorems -Basic notions of concentration inequalities (MacDiarmid, Hoeffding, Bernstein)	Chapter 2 Figures 2.1 and 2.2
October 14	Linear Least-squares regression -Guarantees in the fixed design settings (simple in closed-form) -Ridge regression: dimension independent bounds -Guarantees in the random design settings -Lower bound of performance	Chapter 3 Figures 3.1, 3.2 and 3.3
October 28 (no class on October 21)	Empirical risk minimization -Convexification of the risk -Risk decomposition -Estimation error: finite number of hypotheses and covering numbers -Rademacher complexity -Penalized problems	Chapter 4
November 4	Optimization for machine learning -Gradient descent -Stochastic gradient descent -Generalization bounds through stochastic gradient descent	Chapter 5 Figures 5.1, 5.2 and 5.3
November 18	Local averaging techniques -Partition estimators -Nadaraya-Watson estimators -K-nearest-neighbors -Universal consistency	Chapter 6 Figure 6.5 (only for k-nn)
November 25	Kernel methods -Kernels and representer theorems -Algorithms -Analysis of well-specified models -Sharp analysis of ridge regression -Universal consistency	Chapter 7 Figure 7.3
December 2	Model selection -L0 penalty -L1 penalty -High-dimensional estimation	Chapter 8 Figure 8.2 (only for two dimensions d = 2^8)
December 9 Salle des Actes (45, rue d'Ulm)	Neural networks -Single hidden layer neural networks - Estimation error - Approximation properties and universality	Chapter 9
December 16	Exam

Evaluation

One written in-class exam, and (very) simple coding assignments (to illustrate convergence results, to be sent to learning.theory.first.principles@gmail.com). For all classes, the coding assignment is to reproduce the experiments shown in the book draft and send only the figures to the address above (which is only use for this purpose, all other enquiries should go to francis.bach@ens.fr).

New this year! For a group of a few volunteers who are good Python coders, the goal will be to reproduce figures and produce code that will be on the book website. Matlab code will be provided to make sure the results are the same. This will replace the coding assignments and will come with bonus in the final grade. Another group can do the same for the Julia language.

New this year! The draft book is almost finished, and I am still looking for feedback (typos, unclear parts). Please help! (with some bonus in the final grade).