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