Geodesic
On the relationship between differentiability conditions and existence of a strong gradient
Matematičeskie zametki, Tome 77 (2005) no. 1, pp. 93-98 
G. G. Oniani. On the relationship between differentiability conditions and existence of a strong gradient. Matematičeskie zametki, Tome 77 (2005) no. 1, pp. 93-98. http://geodesic.mathdoc.fr/item/MZM_2005_77_1_a7/
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     title = {On the relationship between differentiability conditions and existence of a strong gradient},
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     url = {http://geodesic.mathdoc.fr/item/MZM_2005_77_1_a7/}
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Voir la notice de l'article provenant de la source Math-Net.Ru

It is proved that for any $n\geqslant2$ there exists continuous function $f\colon\mathbb R^n\to\mathbb R$ which is differentiable almost everywhere, but has no strong gradient almost everywhere.

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[2] Dzagnidze O., “A necessary and sufficient condition for differentiability of functions of several variables”, Proc. A. Razmadze Math. Inst., 123 (2000), 23–29 | MR | Zbl

[3] Saks S., Teoriya integrala, M., 1949 | Zbl