Yudasril, et al. “Sifat Kemonotonan Barisan Trapezoid Sum Dari Kelas Fungsi Nonkonveks Dan Nonkonkaf”. Limits: Journal of Mathematics and Its Applications, vol. 19, no. 1, May 2022, pp. 49-63, https://journal.its.ac.id/index.php/limits/article/view/5346.