The best courses for "subfield of computer science and of mathematics", theoretical computer science.

Machine Learning

Functional Programming Principles in Scala

Functional Programming in Haskell: Supercharge Your Coding

by University of Glasgow

Quantum Mechanics and Quantum Computation

by University of California, Berkeley

Discrete Optimization

Approximation Algorithms Part I

Algorithmic Toolbox

Functional Program Design in Scala

CS For All: Introduction to Computer Science and Python Programming

by Harvey Mudd College

Computing: Art, Magic, Science - Part II

by ETH Zurich

Artificial Intelligence (AI)

by Columbia University

Aplicaciones de la Teoría de Grafos a la Vida Real (I)

by Universitat Politècnica de Valencia

Greedy Algorithms, Minimum Spanning Trees, and Dynamic Programming

How to Code: Complex Data

by University of British Columbia

Algorithmic Thinking (Part 1)