@techreport{TR-IC-09-45,
   number = {IC-09-45},
   author = {Jorge Stolfi},
   title = {{A Two-Phase Model for Wikipedia Growth}},
   month = {November},
   year = {2009},
   institution = {Institute of Computing, University of Campinas},
   note = {In English, 17 pages.
    \par\selectlanguage{english}\textbf{Abstract}
       The  number of articles $N(t)$ in Wikipedia is quite accurately
       modeled as a function of time $t$ by two exponential regimes or
       phases,  with  a  relatively  sharp  transition over a one-year
       period centered on Janary 2006. The first regime has a positive
       rate  constant  $R_1  =  +0.00217$  per day, corresponding to a
       doubling  time  of  about  10.5 months. The second regime has a
       negative rate constant $R_2 = -0.000407$ per day, corresponding
       to  a  halving time of about 4.5 years. The model predicts that
       $N(t)$  will  tend  to  a  finite  limit  of  about 5.9 million
       articles.   We   advance   some   possible   explanations   and  
       implications of the negative rate.
  }
}