On Jacobson's lemma and Drazin invertibility. (Q963823)

In this note the authors answer a question of Patrico and de Costa to note that Jacobson's lemma extends to various abstract versions of ``Drazin invertibility''. They claim that Jacobson's lemma holds for Drazin invertibility: as a preliminary they make an observation about polynomials. In a general ring a polynomial is only allowed to have integer coefficients, while if \(A\) is a real or a complex algebra then the coefficients may also be real or complex. Section 3 is quasinilpotents and Section 4 is quasipolars.