Sequential-Access FM-Indexes
  Previous authors have shown how to build FM-indexes efficiently in external memory, but querying them efficiently remains an open problem. Searching na\"{i}vely for a pattern $P$ requires (\Theta (|P|)) random access. In this paper we show how, by storing a few small auxiliary tables, we can access data only in the order in which they appear on disk, which should be faster. 
Travis Gagie
cs.DS