Bellen, A., and Gheorghe 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
.