[en] In this paper the SU(4) color models represent possible alternatives to the standard model. These theories replace the color gauge group SU(3)_c by SU(4)_c. The SU(4)_c symmetry is assumed to be broken which leads to the SM as the effective low energy theory. An interesting feature of these theories is that the SU(4) symmetry breaking scale can be very low, and may be approximately at the same scale as the electroweak symmetry breaking. These models imply the existence of exotic electrically charged ±1/2 fermions. We examine various SU(4)_c models to explore the circumstances under which these models can have the exotic charged fermions in the interesting mass range m approx-gt 100 GeV

[en] We define the notion of a pendulum on a surface of constant curvature and study the motion of a mass at a fixed distance from a pivot. We consider some special cases: first a pivot that moves with constant speed along a geodesic, and then a pivot that undergoes acceleration along a fixed geodesic