Bellen, A., and G. Micula. “Spline Approximations for Neutral Delay Differential Equations”. Rev. Anal. Numér. Théor. Approx., vol. 23, no. 2, Aug. 1994, pp. 117-25, https://ictp.acad.ro/jnaat/journal/article/view/1994-vol23-no2-art1.