Minh Le Nguyen Nguyen Thanh Tri Akira Shimazu Subtree Mining for Question Classification Problem. 1695-1700 2007 conf/ijcai/2007 IJCAI http://dli.iiit.ac.in/ijcai/IJCAI-2007/PDF/IJCAI07-274.pdf db/conf/ijcai/ijcai2007.html#NguyenNS07