[1] A. M. Bruckner С. Goffman: 
The boundary behavior of real functions in the upper half plane. Revue Roum. Math, pures et appl. 11 (1966), 507-518. 
MR 0206173 
[2] С. Goffman W. T. Sledd: 
Essential cluster sets.  J. London Math. Soc. I, Ser. 2 (1969), 295-302. 
MR 0268384 
[4] A. Khintchine. : 
Recherches sur la structure des fonctions measurables. Fund. Math. 9 (1927), 212-279. 
DOI 10.4064/fm-9-1-212-279 
[5] L. Zajíček: 
On the intersection of the sets of the right and left internal approximate derivatives. Czech. Math. J. 23 (98) (1973), 629-634. 
MR 0323969