The ground state and the magnetization process of the body-centered tetragonal (bet) lattice with Ising spins up to the third neighbor interactions (J,, J,, J,) are investigated by the method of inequalities. Eight types of spin ordering are found of which three are new compared to the case J 3 =0. The magnetization process at T=O is determined and classified in several classes according to combinations of signs and magnitudes of J,, J, and J,. The system with J 8 =0 is topologically reduced to the linear chain and the system with J,=O and that with J,=J, to the square lattice with the 1irst. and the second neighbor interactions. The cause of degeneracies appearing in both cases is clarified by the reduction to lower dimensions. The three-dimensional character is realized only by. taking J, into account. Comparison of the ground states between the case of the ~sing model and the case of the classical Heisenberg model is made in the case where the magnetic field is zero. § I. IntroductionThe studies of spin orderings in the ground state of a spin system with several neighbor interactions are important in connection with phase transitions and the magnetization process. Review of the ground state (other than ferroand antiferromagnetic states) of classical spin systems (Ising and classical Heisenberg model) for several crystal lattices is given shortly. We denote the k-th neighbor interaction by Jk.The ground state and the magnetization process in: the linear chain were discussed by Oguchi,I> Morita and Horiguchi, 2 > Katsura and Narita, 8 > Bandaru,Angelescu anq Nenciu/> and Morita. 5 > For the square lattice with J 1 and J 2 they were discussed by Domb and Potts, 6 > Fan and Wu/> Kanamori, 8 > Horiguchi and Morita, 9 > and Kaburagi and Kanamori. 10 > Ter Haar and Lines 11 > discussed the ground state of sc, bee, fcc, bet lattices with several neighbor interactions and anisotropy in the case where the magnetic field is zero by assuming the sublattice structure. The ground state in the magnetic field of sc, bee, fcc, bet with J 1 and J 2 were obtained by Kanamori 8 > by the method of inequalities. The magnetization process in the J 3 results is expressed in phase diagrams 11y Fig. 1. Body-centered tetragonal lattice.