Simultaneous approximation and global smoothness preservation
C. Cottin, H.H. Gonska, in: Circolo Matematico di Palermo (Ed.), Proceedings of the Second International Conference in Functional Analysis and Approximation Theory, 1993.

Directional extensions of univariate operators and Boolean sum approximation
C. Cottin, Analysis 13 (1993) 185–200.

Directional K- Functionals
C. Cottin, Results in Mathematics 24 (1993) 211–221.