On the approximation of the values of exponential function and logarithm by algebraic numbers
arXiv:math/0002047
Abstract
A sharp explicit estimate is proved for the difference when and are nonzero algebraic numbers.
17 pages. See also http://www.math.jussieu.fr/~miw/articles/ps/Nesterenko.ps and http://www.math.jussieu.fr/~nesteren/