Transactions of the American Mathematical Society, Vol. 302, No. 1 (Jul., 1987), pp. 371-381 (11 pages) Approximation by proximal normals to the Clarke generalized subdifferential for a distance ...
Merton, Robert C., and Paul A. Samuelson. "Fallacy of the Log-Normal Approximation to Optimal Portfolio Decision Making over Many Periods." Journal of Financial Economics 1 (May 1974): 67–94.