  Series expansion based on the flow equation method is employed to study the
zero temperature properties of the spin-1/2 J1-J2-J3 antiferromagnet in two
dimensions. Starting from the exact limit of decoupled plaquettes in a
particular generalized J1-J2-J3 model we analyze the evolution of the ground
state energy and the elementary triplet excitations in powers of all three
inter-plaquette couplings up to fifth order. We find the plaquette phase to
remain stable over a wide range of exchange couplings and to connect
adiabatically up to the case of the plain J1-J2-J3 model, however not to the
J1-J2 model at J3=0. Besides confirming the existence of such a phase, recently
predicted by Mambrini, et al. [Phys. Rev. B 74, 144422 [2006)], we estimate its
extent by Dlog-Pade analysis of the critical lines that result from closure of
the triplet gap.
