Geometric Characterizations of C1 Manifold in Euclidean Spaces by   Tangent Cones
  A remarkable and elementary fact that a locally compact set F of Euclidean space is a smooth manifold if and only if the lower and upper paratangent cones to F coincide at every point, is proved. The celebrated von Neumann's result (1929) that a locally compact subgroup of the general linear group is a smooth manifold, is a straightforward application. A historical account on the subject is provided in order to enrich the mathematical panorama. Old characterizations of smooth manifold (by tangent cones), due to Valiron (1926, 1927) and Severi (1929, 1934) are recovered; modern characterizations, due to Gluck (1966, 1968), Tierno (1997), Shchepin and Repovs (2000) are restated. 
Francesco Bigolin and Gabriele H. Greco
math.DG