Balázs, M., and G. Goldner. “On Approximate Solving by Sequences the Equations in Banach Spaces”. Anal. Numér. Théor. Approx., vol. 8, no. 1, Feb. 1979, pp. 27-31, https://ictp.acad.ro/jnaat/journal/article/view/1979-vol8-no1-art2.