Popov, V.A. “Direct and Converse Theorems for Spline Approximation With Free Knots in \(L_p\)”.
Anal. Numér. Théor. Approx. 5, no. 1 (February 1, 1976): 69–78. Accessed April 11, 2025.
https://ictp.acad.ro/jnaat/journal/article/view/1976-vol5-no1-art7.