thanks, now it works perfectly.
The Bose-Hubbard model describes Bose particles on a lattice that exhibit a repulsive interaction (described by the part I put into H) and a hopping to neighbouring lattice sites (described by V). As done in this paper (http://arxiv.org/abs/1401.0680), I'm interested in the phase change this model undergoes if one varies the hopping strength J. One way of obtaining the necessary terms in Kato pertubation theory is the process-chain approach (briefly described in the paper). These expressions are commenly calculated numerically, however at the phase boundary, the perturbation series diverges so that, it is insufficient to obtain just the first few orders. I'm currently working on replacing the graphical extrapolation, which is used in the paper. To do so, I wanted to get some analytical results for the first few orders. I did some calculations by hand, but figured that I might safe a lot of time using Maple.
Choosing particular terms, was mainly to evaluate their contribution and I was trying to manoeuvre around the problem with the projections in perturbation theory (http://www.mapleprimes.com/questions/201948-How-Can-One-Use-Projectors-In-Pertubation-Theory).
As soon as I have a version, which is satisfactory, I could post it in either this post or the one about projections in perturbation theory.