Kolumbán, I. “Über Ein Abstraktes Maximumprinzip: On an Abstract Maximum Principle”. Rev. Anal. Numér. Théorie Approximation 3, no. 1 (February 1, 1974): 37–46. Accessed April 20, 2024. https://ictp.acad.ro/jnaat/journal/article/view/1974-vol3-no1-art6.