Groze, S., and B. Janko. “Application of Iterative Methods for Solving Operator Equations and Improving Convergence Conditions”. Anal. Numér. Théor. Approx., vol. 6, no. 1, Feb. 1977, pp. 15-21, https://ictp.acad.ro/jnaat/journal/article/view/1977-vol6-no1-art3.