Answer to an open problem proposed by E Barkai and J Klafter

In a negative answer to the open problem proposed by E Barkai andJ Klafter, it is proved that the theory presented by Amblard et al is not consistent with the GER. We suggest applying a theory in terms of a fractional Fokker?Planck equation to model the experiment measured by Amblard et al. The result obtained is consistent with the statement by Amblard et al (1996 Phys. Rev. Lett. 77 4470) that the numerical pre-factors of the power law would be modified by local geometry and are not exact.