no. 3
On restricted derivative approximation
Compositio Mathematica, Tome 74 (1990) no. 3, pp. 327-331. 
@article{CM_1990__74_3_327_0,
     author = {Yang, Zi-Xiao and Li, Bu-Xi},
     title = {On restricted derivative approximation},
     journal = {Compositio Mathematica},
     pages = {327--331},
     publisher = {Kluwer Academic Publishers},
     volume = {74},
     number = {3},
     year = {1990},
     mrnumber = {1055699},
     zbl = {0697.41002},
     language = {en},
     url = {http://archive.numdam.org/item/CM_1990__74_3_327_0/}
}
TY  - JOUR
AU  - Yang, Zi-Xiao
AU  - Li, Bu-Xi
TI  - On restricted derivative approximation
JO  - Compositio Mathematica
PY  - 1990
SP  - 327
EP  - 331
VL  - 74
IS  - 3
PB  - Kluwer Academic Publishers
UR  - http://archive.numdam.org/item/CM_1990__74_3_327_0/
LA  - en
ID  - CM_1990__74_3_327_0
ER  - 
%0 Journal Article
%A Yang, Zi-Xiao
%A Li, Bu-Xi
%T On restricted derivative approximation
%J Compositio Mathematica
%D 1990
%P 327-331
%V 74
%N 3
%I Kluwer Academic Publishers
%U http://archive.numdam.org/item/CM_1990__74_3_327_0/
%G en
%F CM_1990__74_3_327_0
Yang, Zi-Xiao; Li, Bu-Xi. On restricted derivative approximation. Compositio Mathematica, Tome 74 (1990) no. 3, pp. 327-331. http://archive.numdam.org/item/CM_1990__74_3_327_0/

[1] Roulier, J.A., Best Approximation to Functions with Restricted derivate, J. Approximation Theory 17 (1976), 344-347. | MR | Zbl

[2] Feinerman, R.P. and Newman, D.J., "Polynomial Approximation", Williams and Wilkins, Baltimore 1974. | MR | Zbl

[3] Leviatan, D., The behavior of the derivatives of the algebraic polynomials of best approximation, J. Approximation Theory 35 (1982), 169-176. | MR | Zbl