Online tutorials and reading in mathematics

Cut-the-Knot: Interactive Mathematics Miscellany and Puzzles by the late Alexander Bogomolny

imaginary.org: open mathematics, “initiated” at the Mathematisches Forschungsinstitut Oberwolfach

Math Counterexamples by Jean-Pierre Merx

Theorem of the Day by Robin Whitty

YouTube channels

• At Michael Penn Math, Michael Penn of Randolph College in Lynchburg, Virginia, delivers clear, intuition-creating solutions of problems from various mathematical competitions.

• Aleph 0

• eigenchris

• 3Blue1Brown (Grant Sanderson)

• Mathologer (Burkard Polster of Monash University in Melbourne, Australia)

• Numberphile