  The exact renormalization group is used to study the RG flow of quantities in
field theories. The basic idea is to write an evolution operator for the flow
and evaluate it in perturbation theory. This is easier than directly solving
the differential equation. This is illustrated by reproducing known results in
four dimensional $\phi^4$ field theory and the two dimensional Sine-Gordon
theory. It is shown that the calculation of beta function is somewhat
simplified. The technique is also used to calculate the c-function in two
dimensional Sine-Gordon theory. This agrees with other prescriptions for
calculating c-functions in the literature. If one extrapolates the connection
between central charge of a CFT and entanglement entropy in two dimensions, to
the c-function of the perturbed CFT, then one gets a value for the entanglement
entropy in Sine-Gordon theory that is in exact agreement with earlier
calculations (including one using holography) in arXiv:1610.04233.
