  We express the nonlocal BMS charges of a free massless Klein-Gordon scalar
field in 2+1 in terms of the Green functions of the polyharmonic operators.
Using the properties of these Green functions, we are able to discuss the
asymptotic behaviour of the fields that ensures the existence of the charges,
and prove that one obtains a realization of the 2+1 BMS algebra in canonical
phase space. We also discuss the transformations in configuration space, and
show that in this case the algebra closes only up to skew-symmetric
combinations of the equations of motion. The formulation of the charges, in
terms of Green functions, opens the way to the generalization of the formalism
to other dimensions and systems.
