Precup, R. “Convexity and Quadratic Monotone Approximation in Delay Differential Equations”. Rev. Anal. Numér. Théor. Approx., vol. 30, no. 1, Feb. 2001, pp. 89-93, https://ictp.acad.ro/jnaat/journal/article/view/2001-vol30-no1-art12.