http://doi.acm.org/10.1145/511754.511767 Alan F. Smeaton C. J. van Rijsbergen The Nearest Neighbour Problem in Information Retrieval: An Algorithm Using Upperbounds. 83-87 SIGIR1981/P083.pdf 1981 conf/sigir/81 SIGIR db/conf/sigir/sigir81.html#SmeatonR81