Nicola, A., and C. Popa. “Preconditioning by an Extended Matrix Technique for Convection-Diffusion-Reaction Equations”. Rev. Anal. Numér. Théor. Approx., vol. 37, no. 2, Aug. 2008, pp. 181-90, https://ictp.acad.ro/jnaat/journal/article/view/2008-vol37-no2-art10.