Precup, Radu. “ Quasiconvex Functions of Higher Order and the Behaviour of Some Nonlinear Functionals”. Rev. Anal. Numér. Théor. Approx., vol. 21, no. 2, Aug. 1992, pp. 191-3, https://ictp.acad.ro/jnaat/journal/article/view/1992-vol21-no2-art11.