  In the objective of studying concentration and oscillation properties of
eigenfunctions of the discrete Laplacian on regular graphs, we construct a
pseudo-differential calculus on homogeneous trees, their universal covers. We
define symbol classes and associated operators. We prove that these operators
are bounded on L^2 and give adjoint and product formulas. Finally we compute
the symbol of the commutator of a pseudo-differential operator with the
Laplacian.
