Wolfram MathWorld is an online mathematics encyclopedia covering algebra, calculus, geometry, and related topics with definitions and formulas.
Online encyclopedia of mathematical definitions and results.
PlanetMath is a collaborative online encyclopedia of mathematics entries written and peer-reviewed by community members.
Problems and solutions from the International Mathematics Competition for university students.
Large interactive collection of mathematical problems and proofs.
Online database of mathematical objects used in number theory and related fields.
Online collection and database of mathematical proofs organized by topic.
A personal website with notes on mathematics and related topics.
Russian portal for mathematics education containing problems, methodologies, and reference materials.
Online repository of digitized mathematical literature and papers.
Archive of mathematics olympiad problems (not physics); not relevant to physics students.