Can nonextremal Reissner-Nordstrom black hole become extremal by   assimilating infalling charged particle and shell?
  By using the gedanken experiments suggested by Bekenstein and Rosenzweig, we have shown that nonextremal Reissner-Nordstrom black hole cannot turn into extremal one by assimilating infalling charged particle and charged spherical shell. 
Bin Wang, Ru-Keng Su, P.K.N.Yu and E.C.M.Young