1. Letter dated July 31, 1987, to Bruce Piper.
2. C. de Boor and K. Höllig, Approximation Power of smooth bivariate pp functions, Math.Z. 197 (1988), 343–363.
3. C.K. Chui and M.J. Lai, On bivariate vertex splines, in Multivariate Approximation III W. Schempp and K. Zeller eds., Birkhäuser, Basel, 1985, 84–115.
4. C.K. Chui M. Lai, On bivariate super vertex splines, CAT Report 164, March 1988, Texas AM University, College Park TX.
5. Adel Kh. Ibrahim and Larry L. Schumaker, Super Spline Spaces of Smoothness r and Degree d ≥ 3r + 2, ms, (1988).