[1] Becker M., Kucharski D., Nessel R. Ј.:
Global approximation theorems for the Szasz-Mirakjan operators in exponential weight spaces. In: Linear Spaces and Approximation (Proc. Conf. Oberwolfach, 1977), Birkhäuser Verlag, Basel ISNM, 40 (1978), 319-333.
MR 0499919
[2] De Vore R. A., Lorentz G. G.:
Constructive Approximation. Springer-Verlag, Berlin, 1993.
MR 1261635