Characterisation theorem for best polynomial spline approximation with free knots
Author: Sukhorukova, Nadezda.  Published in: Transactions of the American Mathematical Society, v. 369  no. 9, pp. 6389- 6405  Date: 2017
ISSN:  0002-9947,  EISSN:  1088-6850  DOI:  10.1090/tran/6863