  We show that Busemann functions on a smooth, non-compact, complete,
boundaryless, connected Riemannian manifold are viscosity solutions with
respect to the Hamilton-Jacobi equation determined by the Riemannian metric and
consequently they are locally semi-concave with linear modulus. We also
analysis the structure of singularity sets of Busemann functions. Moreover we
study barrier functions, which are analogues to Mather's barrier functions in
Mather theory, and provide some fundamental properties. Based on barrier
functions, we could define some relations on the set of lines and thus classify
them. We also discuss some initial relations with the ideal boundary of the
Riemannian manifold.
