Dimensions of identity matrix | Matrices | Precalculus | Khan Academy
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Chapter 04.01: Lesson: What is an identity matrix?
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Identity Matrix - Interactive 3D Graphics
In linear algebra, the identity matrix, or sometimes ambiguously called a unit matrix, of size n is the n × n square matrix with ones on the main diagonal and zeros elsewhere. It is denoted by In, or simply by I if the size is immaterial or can be trivially determined by the context. Less frequently, some mathematics books use U or E to represent the identity matrix, meaning "unit matrix" and the German word "Einheitsmatrix", respectively.