[1] GOFFMAN C., NEUGEBAUER C. J.: On approximate derivatives. Proc. Amer. Math. Soc., 11, 1960, 962-966. MR 0118792

[2] MIŠÍK L.: Bemerkungen über approximаtive Ableitung. Mаt. čаsop., 19, 1969, 283-291. MR 0302834

[3] MIŠÍK L.: A note concerning а pаper by L. E. Snyder. Mаt. časop., 19, 1969, 188-191. MR 0302829

[4] PREISS D.: Approximаte derivаtives аnd Bаire clаsses. Czech. Math. Jouгn., 21 (96), 1971, 373-382. MR 0286951

[5] SNYDER L. E.: Approximate Stolz angle limits. Proc. Ameг. Math. Soc., 17, 1966, 416-422. MR 0188383 | Zbl 0158.05102

[6] TOLSTOV G.: Sur la dérivée approximativе еxactc. Mat. Sb. 4 (46), 1938, 499-504.

[7] ZAHORSKI Z.: Sur la classе dе Bairе dеs dérivéеs approximatives ď une fonction quelconque. Annal. Soc. Polon. Math., 21, 1948, 306 - 323. MR 0031034