Kostova, V. A. “Approximation of Non-Negative Convex Functions by Polynomials With Positive Coefficients in the Hausdorff Metric”. Anal. Numér. Théor. Approx., vol. 12, no. 2, Aug. 1983, pp. 141-9, https://ictp.acad.ro/jnaat/journal/article/view/1983-vol12-no2-art5.