We consider a Hamiltonian of a system of two fermions on a three-dimensional lattice Z
3 with special potential

. The corresponding Shrödinger operator
H(
k) of the system has an invariant subspac
L-123(T3) , where we study the eigenvalues and eigenfunctions of its restriction
H-123(k). Moreover, there are shown that
H-123(k1, k2, π) has also infinitely many invariant subspaces

, where the eigenvalues and eigenfunctions of eigenvalue problem

are explicitly found.