  The ground-state properties of the S=1/2 frustrated Heisenberg spin chain
with interactions up to fourth nearest neighbors are investigated by the
exact-diagonalization method and density matrix renormalization group method.
Our numerical calculations clarify that the ferrimagnetic state is realized in
the ground state in spite of the fact that a multi-sublattice structure in the
shape of the system is absent. We find that there are two types of
ferrimagnetic phases: one is the well-known ferrimagnetic phase of the
Lieb-Mattis type and the other is the nontrivial ferrimagnetic phase that is
different from that of the Lieb-Mattis type. Our results suggest that a
multi-sublattice structure of the shape is not necessarily required for the
occurrence of ferrimagnetism.
