1. Interval-functions;Gleyzal, André;Duke Math. J.,1941
2. On approximate derivatives;Goffman, Casper;Proc. Amer. Math. Soc.,1960
3. The exact Peano derivative;Oliver, H. William;Trans. Amer. Math. Soc.,1954
4. Sur la première dérivée;Zahorski, Z.;Trans. Amer. Math. Soc.,1950