Summary: This article is dedicated to Professor Constantin Popa – math teacher, professor, researcher, colleague, and friend. Our goal is to present some of the most important aspects of his scientific activity, as well as some traits that made both him a remarkable scientist and colleague, with high standards of professionalism. He published papers on preconditioning techniques for finite element method and finite differences discretizations for boundary value problems, iterative methods for least-squares problems for linear systems of equalities and inequalities (projection algorithms, esp. Kaczmarkz-like algorithms), algebraic reconstruction techniques in computerized tomography, inverse problems regularization techniques and methods for approximating the minimal norm solution of first-kind integral equations.