  We study the effect of an optical lattice (OL) on the ground-state properties
of one-dimensional ultracold bosons with three-body attraction and two-body
repulsion, which are described by a cubic-quintic Gross-Pitaevskii equation
with a periodic potential. Without the OL and with a vanishing two-body
interaction term, soliton solutions of the Townes type are possible only at a
critical value of the three-body interaction strength, at which an infinite
degeneracy of the ground-state occurs; a repulsive two-body interaction makes
such localized solutions unstable. We show that the OL opens a stability window
around the critical point when the strength of the periodic potential is above
a critical threshold. We also consider the effect of an external parabolic
trap, studying how the stability of the solitons depends on matching between
minima of the periodic potential and the minimum of the parabolic trap.
