  In this paper, the form of the indirect interaction between local impurities
in flakes and in graphene ring is analytically investigated. We calculate this
characteristic in the framework of the tight-binding model for a finite
nanocarbon system with periodic boundary conditions. A pronounced dependence of
the bond value on the impurity position inside the graphene sample is found,
which is due to quantum size effects. The influence of the flake size on the
value of the indirect interaction constant is also studied.
