[1] F. S. De Blasi: 
On the differentiability of multifunctions. Pacif. J. Math. 66 (1976), 67-81. 
MR 0445534 | 
Zbl 0348.58004[2] M. Boudourides J. Schinas: 
The mean value theorem for multifunctions. Bull. Math. Roumanie 25 (1981), no 2, 129-141. 
MR 0636833[3] M. Boudorides J. Schinas: 
Higher order differentiability of multifunctions. Nonlin. Anal. 5 (1981), no 5, 509-516. 
MR 0613059[4] P. Kranz: 
A metrizable cancellative semigroup without translation invariant metric. Proc. Amer. Math. Soc. 66 (1977), no 1, 17-19. 
MR 0450452 | 
Zbl 0367.22003