ncatlab.org
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nLab is a collaborative online encyclopedia of higher mathematics, focused on category theory, higher categories, algebraic geometry, and related areas. It provides peer-edited definitions, explanations, and interconnected pages (including schemes) to explore concepts and their relationships.
More resources on Schemes
Ravi Vakil's FOAG Notes
Free comprehensive notes on foundations including schemes
The Stacks Project
Collaborative textbook on algebraic geometry
What is a scheme?
Overview of schemes in algebraic geometry
Algebraic Geometry
Learn schemes with Prof. Borcherds' Algebraic Geometry course! Explore this foundational topic via lectures on YouTube.
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia from Wolfram Research that provides in-depth, accessible articles on a wide range of topics. Each entry includes definitions, explanations, formulas, examples, and visuals, with cross-references—covering both foundational subjects and advanced topics like schemes in algebraic geometry.
ocw.mit.edu
MIT OpenCourseWare provides free, open access to MIT course materials—lecture notes, assignments, exams, and video lectures—from a wide range of disciplines. It includes computer science resources, such as SICP materials for learning Scheme, enabling self-paced study.