⟨ A ( R → → n , R → → m ) ⟩ = ∫ ∫ d R → → N d R → → n d R → → m d R → → 0 G ( R → → N , R → → n ; N − − n ) G ( R → → n , R → → m ; n − − m ) A ( R → → n , R → → m ) ∫ ∫ d R → → N d → → R 0 G ( R → → N , R → → 0 ; N )