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A free resource based on Eric Weissteinn CRC Concise Encyclopedia of Mathematics.
Ask Dr. Math
A question and answer service for math students and their teachers. A searchable archive is available by level and topic, as well as summaries of Frequently Asked Questions.
PRIME Mathematics Encyclopedia
Comprehensive mathematics encyclopedia including information on a wide range of topics approachable from pre-collegiate to advanced graduate and research levels.
The Problems Library is an archive of Math Forum’s "Problem of the Week" organized by topic and rated for difficulty level.
An open website of problems and puzzles, online mentoring, research, team problem solving, collaborations, and professional development.