ocw.mit.edu
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MIT OpenCourseWare (ocw.mit.edu) offers free, openly accessible MIT course materials across many subjects, including advanced physics. You can access lecture notes, problem sets, exams, and sometimes video lectures for string theory–related courses, enabling self-paced study.
More resources on String Theory
StringWiki
Community wiki for string theory resources
nLab - String Theory
Mathematical perspective and references
Superstringtheory.com
Comprehensive intro site with basics and visuals
String Theory and Holographic Duality
Explore the fascinating world of string theory and holographic duality with Pavel Kroupa. Learn the fundamentals and latest advancements in this course!
String Theory I
Learn the fundamentals of string theory with Leonard Susskind's accessible "String Theory I" course. Explore the universe's deepest secrets!
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia from Wolfram Research offering detailed, rigorously written articles on a wide range of mathematical topics, with definitions, formulas, diagrams, and examples. It covers many concepts relevant to string theory—such as topology, differential geometry, algebra, and special functions—providing foundational explanations useful for theoretical physics.