A Note on Alberti's Luzin-Type Theorem for Gradients

We give a “soft” proof of Alberti’s Luzin-type theorem in [1] (G. Alberti, A Lusintype theorem for gradients, J. Funct. Anal. 100 (1991)), using elementary geometric measure theory and topology. Applications to the $C^2$-rectifiability problem are also discussed.