Tatsuya Akutsu Hideo Bannai Satoru Miyano Sascha Ott On the complexity of deriving position specific score matrices from positive and negative sequences. 676-685 2007 155 Discrete Applied Mathematics 6-7 http://dx.doi.org/10.1016/j.dam.2004.10.011 db/journals/dam/dam155.html#AkutsuBMO07