A matrix satisfying $A^2 = A$ is called an idempotent matrix.
Let $\lambda$ be an eigenvalue of $A$ with corresponding non-zero eigenvector $v$:
$$Av = \lambda v$$Multiplying by $A$:
$$A^2 v = A(Av) = A(\lambda v) = \lambda(Av) = \lambda(\lambda v) = \lambda^2 v$$Since $A^2 = A$:
$$Av = \lambda^2 v \implies \lambda v = \lambda^2 v$$ $$(\lambda^2 - \lambda) v = 0$$Because $v \ne 0$:
$$\lambda^2 - \lambda = 0 \implies \lambda(\lambda - 1) = 0 \implies \lambda = 0 \text{ or } \lambda = 1$$आव्यूह $A^2 = A$ को वर्गसम आव्यूह (Idempotent matrix) कहा जाता है।
यदि $\lambda$ इसका आइगेन मान और $v$ आइगेन सदिश है:
$$Av = \lambda v \implies A^2 v = \lambda^2 v$$चूंकि $A^2 = A$ है:
$$\lambda v = \lambda^2 v \implies \lambda^2 - \lambda = 0 \implies \lambda(\lambda - 1) = 0$$इससे $\lambda = 0$ अथवा $\lambda = 1$ प्राप्त होता है।