Gavrea, I., and I. Rașa. “Remarks on Some Quantitative Korovkin-Type Results”. Rev. Anal. Numér. Théor. Approx., vol. 22, no. 2, Aug. 1993, pp. 173-6, https://ictp.acad.ro/jnaat/journal/article/view/1993-vol22-no2-art6.