ncatlab.org
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nLab is a collaborative online wiki and reference for higher mathematics (notably category theory, higher algebra, and topology) used by researchers and students. It contains in-depth articles, definitions, and discussions on topics including smooth maps, differential geometry, manifolds, and related concepts.
More resources on Smooth Maps
nLab: Smooth map
Comprehensive entry on smooth maps in higher category theory and topology context
mathworld.wolfram.com
Wolfram MathWorld is a comprehensive online mathematics encyclopedia offering in-depth articles on mathematical concepts, definitions, theorems, formulas, and worked examples. It spans a wide range of topics—from algebra and calculus to geometry and topology—with well-organized entries, diagrams, and references for further exploration.
ocw.mit.edu
MIT OpenCourseWare (OCW) provides free, openly licensed MIT course materials. It offers lecture notes, assignments, exams, and sometimes video lectures from a wide range of courses, including mathematics topics relevant to smooth maps.