Ron Breukelaar Erik D. Demaine Susan Hohenberger Hendrik Jan Hoogeboom Walter A. Kosters David Liben-Nowell Tetris is hard, even to approximate. 41-68 2004 14 Int. J. Comput. Geometry Appl. 1-2 http://dx.doi.org/10.1142/S0218195904001354 db/journals/ijcga/ijcga14.html#BreukelaarDHHKL04