ncatlab.org
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nLab is a collaborative wiki for higher mathematics and related areas. It hosts extensive entries on topics like sheaves, category theory, topology, and higher categories, including definitions, explanations, examples, diagrams, and cross-referenced notes.
More resources on Sheaves
nLab: Sheaf
Detailed overview and references for sheaves in algebraic geometry
Stacks Project
Chapter 12-17 cover sheaves extensively, free comprehensive reference
Algebraic Geometry
Learn about sheaves in algebraic geometry with this course from Angelo Vistoli. Explore the AG.pdf resource now!
Foundations of Algebraic Geometry
Learn the foundations of algebraic geometry with Ravi Vakil's comprehensive course, focusing on sheaves and fundamental concepts.
mathworld.wolfram.com
MathWorld is Wolfram’s extensive online mathematics encyclopedia, offering in-depth articles with definitions, theorems, formulas, and diagrams. It covers a wide range of topics—from algebra and calculus to topology—often including articles on advanced concepts such as sheaves, with extensive cross-references.
ocw.mit.edu
MIT OpenCourseWare provides free, openly licensed course materials from MIT across a wide range of subjects, including lecture notes, syllabi, assignments, exams, and video lectures. Users can browse courses and download resources to study at their own pace.