Gonska, H. H., & Cao, J. .-D. (1999). Approximation by boolean sums of positive linear operators VI: monotone approximation and global smoothness preservation. Rev. Anal. Numér. Théor. Approx., 28(1), 37–61. Retrieved from https://ictp.acad.ro/jnaat/journal/article/view/1999-vol28-no1-art4