Julia Kempe Alexei Kitaev Oded Regev The Complexity of the Local Hamiltonian Problem. 1070-1097 2006 35 SIAM J. Comput. 5 http://epubs.siam.org/SICOMP/volume-35/art_44522.html db/journals/siamcomp/siamcomp35.html#KempeKR06