On non-markovian nature of stock trading
  Using a relationship between the moments of the probability distribution of times between the two consecutive trades (intertrade time distribution) and the moments of the distribution of a daily number of trades we show, that the underlying point process generating times of the trades is an essentially non-markovian long-range memory one. Further evidence for the long-range memory nature of this point process is provided by the powerlike correlation between the intertrade time intervals. The data set includes all trades in EESR stock on the Moscow International Currency Exchange in January 2003 - September 2003 and in Siemens, Commerzbank and Karstadt stocks traded on the Xetra electronic stock exchange of Deutsche Boerse in October 2002. 
Andrei Leonidov