Hojman Symmetry in $f(T)$ Theory
  Today, $f(T)$ theory has been one of the popular modified gravity theories to
explain the accelerated expansion of the universe without invoking dark energy.
In this work, we consider the so-called Hojman symmetry in $f(T)$ theory.
Unlike Noether conservation theorem, the symmetry vectors and the corresponding
conserved quantities in Hojman conservation theorem can be obtained by using
directly the equations of motion, rather than Lagrangian or Hamiltonian. We
find that Hojman symmetry can exist in $f(T)$ theory, and the corresponding
exact cosmological solutions are obtained. We find that the functional form of
$f(T)$ is restricted to be the power-law or hypergeometric type, while the
universe experiences a power-law or hyperbolic expansion. These results are
different from the ones obtained by using Noether symmetry in $f(T)$ theory.
Therefore, it is reasonable to find exact cosmological solutions via Hojman
symmetry.
