Codes for rank modulation have been recently proposed as a means of
protecting flash memory devices from errors. We study basic coding
theoretic problems for such codes, representing them as subsets of the
set of permutations of n elements equipped with the Kendall tau
distance. We derive several lower and upper bounds on the size of
codes. These bounds enable us to establish the exact scaling of the
size of optimal codes for large values of n. We also show the
existence of codes whose size is within a constant factor of the
sphere packing bound for any fixed number of errors. Details of this
work appear in our preprint, arXiv:0908-4094.