by Yee Wei Law - Sunday, 24 December 2023, 11:33 AM
A matrix is a projector if it is 1️⃣ Hermitian and 2️⃣ idempotent [Ber18, Definition 4.1.1].
References
[Ber18]
D. S. Bernstein, Scalar, Vector, and Matrix Mathematics: Theory, Facts, and Formulas - Revised and Expanded Edition, Princeton University Press, 2018. https://doi.org/10.1515/9781400888252.