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MathWorld is Wolfram's comprehensive online mathematics encyclopedia, with in-depth articles on matrices and other linear algebra topics. Each page provides definitions, theorems, formulas, diagrams, worked examples, and links to related concepts and references.
More resources on Matrices
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Paul's Online Math Notes - Linear Algebra
Detailed notes on matrices, free textbook style
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3Blue1Brown Linear Algebra Series
Animated visualizations for matrices and transformations
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MIT OpenCourseWare Linear Algebra
Full course with videos, notes, exams
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Eigenvectors and eigenvalues | Essence of linear algebra
Intuitive explanation of eigenvalues
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The Matrix Equation Ax=b
Gilbert Strang lecture on solving systems with matrices
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Essence of Linear Algebra Chapter 1: Vectors
Visual introduction to vectors, column space, null space