Date: Thu, 09 Jan 2003 From: Giovanni Cicuta Dear Predrag, I am happy your book will appear. I gave a very quick reading to the parts of http://www.cns.gatech.edu/GroupTheory/chapters/GroupTheory.pdf which are closer to my interests. A few insignificant comments and references (mine) are here attached. Yours, Giovanni Color decomposition and color basis. Several papers on QCD by Bern, Dixon, Kosower and others discuss those decompositions . Your old paper on gauge sets, Cvitanovic, Lawres, Scharbach, Nucl.Phys B186, (1981) 165 , is occasionally quoted. A more recent paper is : \bibitem{Duca:2000} V.~Del~Duca, L.~J. Dixon and F.~Maltoni, % Vittorio Del Duca, Lance J. Dixon, Fabio Maltoni . ``New color decompositions for gauge amplitudes at tree and loop level,'' % SLAC-PUB-8294, DFTT-53-99, Oct 1999. 17pp. {\em Nucl. Phys. B \bf 571}, 51 (2000); %51-70 \arXiv{hep-ph/9910563}. Long ago in the paper GAUGE SETS AND 1/N EXPANSION, By E. Ciapessoni, G.M. Cicuta , Published in Nucl.Phys.B219 (1983) 513-523, I found that your diagrammatic methods are very effective to find useful tensor basis to espress sets of gauge invariant amplitudes (as I wrote in the acknowledgment, you corrected a mistake in the first draft of the paper). I think that the works on the subject in the following 20 years were not very new and they did not use the most effective tools. Maybe it would be good to have a remark in your book to the usefulness of the gauge invariant sets described in your paper (1981), their relevance in perturbative QCD, and the usefulness of diagrammatic methods to this problem. References I have no suggestion for references that you should add in your book. As to references that you may add, there are very many. But let me worn you : an author not quoted is unhappy, but he is more unhappy if very many papers similar to his own are quoted. Then as you add references, it is likely that more people will be unhappy. I have a few old papers where I did use your diagrammatic methods , occasionally I did a very a straightforward elaboration and I often stressed the usefulness of diagrammatic notation and tools. \bibitem{Cicuta:1982} G.M. Cicuta, %(Milan U. & INFN, Milan). ``Topological expansion for SO(n) and SP(2n) gauge theories,'' {\em Lett. Nuovo Cim. \bf 35}, 87 (1982). Here I describe the large-n expansion for gauge theories with SO(n) or Sp(n) groups. The problem had previously been discussed in a more limited way by Canning, Phys.Rev.D12 (1975), 2506. Your diagrammatic techniques are used. The replacement n to -n for gauge invariant quantities of the SO(n) and SP(n) models is noted, as well as your previous work with Kennedy. - HIGH-ENERGY LIMIT AND INTERNAL SYMMETRIES. By G.M. Cicuta, D. Gerundino (Milan U.). Published in Phys.Rev.D29:1258-1266,1984 Most of the new work in this paper is the evaluation of group factors in SU(N) for classes of relevant Feynman graphs. One can perform series summations after decomposing contributions with different quantum numbers (actually contributions belonging to different representations of the group). This is done using projectors. We interacted during the work and you made contributions. In the end, instead of an acknowledgment you preferred a sentence, which appear as reference n.11. - DIAGONALIZATION OF A COLORING PROBLEM (ON A STRIP). By G.M. Cicuta, A. Pavone Published in J.Phys.A22:4921,1989 Here I considered a problem exactly solved by Baxter : evaluating the number of ways of proper coloring (with 3 colors) the bonds of a lattice made by regular exagons. You may view this lattice as made by rectangular bricks, the way brick walls are made. And consider a strip of the brick wall, the projectors of SU(2), here I am using Penrose and your results. The transfer matrix corresponding to a strip of finite width provides an approximation to the exact Baxter result, obtained by a completely different approach.