For matrix A if AA^T = I, I is identity matrix then A is?
(a) Orthagonal matrix
(b) Nilpotent matrix
(c) Idempotent matrix
(d) None of the mentioned
The question was posed to me in an international level competition.
This intriguing question comes from Types of Matrices in chapter Basic Structures: Sets, Functions, Sequences, Sums and Matrices of Discrete Mathematics