Duca, D. I. “Necessary Optimality Criteria in Nonlinear Programming in Complex Space With Differentiability”. Anal. Numér. Théor. Approx., vol. 9, no. 2, Aug. 1980, pp. 163-79, https://ictp.acad.ro/jnaat/journal/article/view/1980-vol9-no2-art2.