Spectra of the difference, sum and product of idempotents
Studia Mathematica 148 (2001), 1-3 MSC: 46C05, 47A10, 47B15. DOI: 10.4064/sm148-1-1
We give a simple proof of the relation between the spectra of the difference and product of any two idempotents in a Banach algebra. We also give the relation between the spectra of their sum and product.