Ullrich, C. “Intervallrechnung über vollständig Schwach Geordneten Vektoiden: Interval Arithmetic over Fully Weakly Ordered Vectoids”. Anal. Numér. Théor. Approx., vol. 9, no. 1, Feb. 1980, pp. 133-50, https://ictp.acad.ro/jnaat/journal/article/view/1980-vol9-no1-art18.