Invertibility of the Sum of Idempotents
J. J. Koliha, Vladimir Rakočević · Linear and Multilinear Algebra · 2002
We study necessary and sufficient conditions for the invertibility of the sum f+g when f and g are idempotents in a unital ring or bounded linear operators in Hilbert or Banach spaces. We describe the relation between the invertibility of f+g and f m g.