Haussmann, W. “Spline-Systeme Und Beste Approximation Von Stetigen Linearen Funktionalen: Spline Systems and Best Approximation of Continuous Linear Functionals”. Anal. Numér. Théor. Approx., vol. 4, no. 1, Feb. 1975, pp. 13-18, https://ictp.acad.ro/jnaat/journal/article/view/1975-vol4-no1-art2.