Collatz, L. “Verkettete Und Freie Approximation Bei Differentialgleichungen: Chained and Free Approximation for Differential Equations”. Rev. Anal. Numér. Théorie Approximation, vol. 3, no. 2, Aug. 1974, pp. 151-60, https://ictp.acad.ro/jnaat/journal/article/view/1974-vol3-no2-art4.