Vladik Kreinovich Chin-Wang Tao Checking identities is computationally intractable NP-hard and therefore human provers will always be needed. 39-49 2004 19 Int. J. Intell. Syst. 1-2 http://dx.doi.org/10.1002/int.10149 db/journals/ijis/ijis19.html#KreinovichT04