The phase flow is the one-parameter group of transformations of phase space

{\bf{p}}(0),{\bf{q}}(0))\longmapsto({\bf{p}}(t),{\ bf{q}}(t)) " alt="g^t{\bf{p}}(0),{\bf{q}}(0))\longmapsto({\bf{p}}(t),{\ bf{q}}(t)) " />,

where {\bf{p}}(t) and {\bf{q}}(t) are solutions of the Hamilton's system of equations corresponding to initial condition {\bf{p}}(0) and {\bf{q}}(0).

Show that \{g^t\} is a group.

Any idea?