Ashok K. Goel Neelam Soundararajan B. Chandrasekaran Complexity in Classificatory Reasoning. 421-425 1987 conf/aaai/1987 AAAI db/conf/aaai/aaai87.html#GoelSC87 http://www.aaai.org/Library/AAAI/1987/aaai87-075.php