Păltănea, Radu. “ The Preservation of the Property of the Quasiconvexity of Higher Order by Bernstein’s Operators”. Rev. Anal. Numér. Théor. Approx., vol. 25, no. 1, Aug. 1996, pp. 195-01, https://ictp.acad.ro/jnaat/journal/article/view/1996-vol25-nos1-2-art19.