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One of the most interesting application of the many-body methods to the
nuclear physics is the calculation of the properties of the nuclear matter,
especially its binding energy. In this paper, we have studied the influence
of $\Delta$-isobar on the properties of the symmetrical nuclear matter using
the lowest order constrained variational (LOCV) method with the $V_{28}$
potential. It is shown that the inclusion of $\Delta$-isobar substantially
affect the saturation properties of the symmetrical nuclear matter.
We have shown that, at low (high) densities, the saturation curve of nuclear
matter is shifted downward (upward). This is due to the fact that the
repulsive effect of the $V_{28}$ potential increases by increasing density.
It is seen that the equation of state of nuclear matter with the $V_{28}$
potential is much harder than those with the $AV_{14}$ potential.\
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