1. D. Ferguson, The question of uniqueness for G. D. Birkhoff interpolation problem, J. Approximation Theory
2 (1969), 1–28.
2. S. Karlin and J. M. Karon, On Hermite-Birkhoff interpolation, J. Approximation Theory
6 (1972), 90–116 (a somewhat expanded version: Poised and non-poised Hermite-Birkhoff interpolations, Indiana Univ. Math. J.
21 (1972), 1131–1170 ).
3. G. G. Lorentz, Birkhoff interpolation and the problem of free matrices, J. Approximation Theory
6 (1972), 283–290.
4. Abstract of [3] in Notices Amer. Math. Soc. (November 1969).
5. G. G. Lorentz, “Birkhoff Interpolation Problem,” Report of the Center for Numerical Analysis, pp. 1–115, The University of Texas at Austin, July 1975.