Haussmann, Werner. “Spline-Systeme Und Beste Approximation Von Stetigen Linearen Funktionalen: Spline Systems and Best Approximation of Continuous Linear Functionals”. Anal. Numér. Théor. Approx. 4, no. 1 (February 1, 1975): 13–18. Accessed May 4, 2024. https://ictp.acad.ro/jnaat/journal/article/view/1975-vol4-no1-art2.