Bifurcation and Global Dynamical Behavior of the $f(T)$ Theory
  Usually, in order to investigate the evolution of a theory, one may find the
critical points of the system and then perform perturbations around these
critical points to see whether they are stable or not. This local method is
very useful when the initial values of the dynamical variables are not far away
from the critical points. Essentially, the nonlinear effects are totally
neglected in such kind of approach. Therefore, one can not tell whether the
dynamical system will evolute to the stable critical points or not when the
initial values of the variables do not close enough to these critical points.
Furthermore, when there are two or more stable critical points in the system,
local analysis can not provide the informations that which one the system will
finally evolute to. In this paper, we have further developed the nullcline
method to study the bifurcation phenomenon and global dynamical behaviour of
the $f(T)$ theory. We overcome the shortcoming of local analysis. And it is
very clear to see the evolution of the system under any initial conditions.
