Thanks for you comments, Gottfried.  The literature on Relaxation is large and long.  Memory is often small and short.  As I explained, I was just lucky in that I had studied these papers carefully long ago, and that a coworker got me "back in the game".  Certainly I always admired those Bloch Hubbard papers, and it was happy thing to re-experience a kind of youthful enthusiasm.

Even the modern topic of Lindblad has a very large literature.  Dave Siminovitch alerted me to the papers by Karl Lendi from the 1980's.  There is also a nice paper by Jim Skinner and coworkers, who reproduce Bloch's result for spin locking and rotating frame relaxation in JCP 102 1995.  I imagine that these authors have confirmed and deepened the scope of applicibility of Bloch's result, but to know for sure would require some digging.  Notations are always a challenge for me anyway.

I agree that the paper is concise but I also claim that it is pretty clear in most places.  I am surprised that many readers are confused about section 2.  To me it seemed a very straightforward application of matrix algebra and certainly I would expect most NMR scientists to have some familararity with that aspect.  And the approach I took allowed me to have free reign on various forms of expression so that I could work out Hubbard's formula in baby steps if you will.  I though it would be of interest to others.

As I worked on this topic I collected my thoughts in a set of notes and I am happy to share these with you if you like.

I thank the reveiwer for taking the time to read and comment on my submission.  The typos in the reference to Bloch escaped my attention as well as some equation numbers and references to them that were mixed up.  There are also incorrect statements regarding the use of the secular approximation and their relation to "bath symmetry" which I knew about but somehow did not change when I edited this for submission to Magnetic Resonance.  Basically the bath symmetry arguement is incorrect: it only takes care of one index.  The edited version will take care of this, with correct equation numbering etc.

On the matter of clairity:  In my experience every reader must dig to some extent, even if one knows the topic fairly well.  Notation is never militarily enforced and some effort is required to go through it.  I expect that if one did dig a little into section 2, they will find the ground soft and easy.  Personally, I found Bengs and Levitt a not altogether "easy read", with unfamilar notation in places that I had to sort out, especially to the manner inwhich they expressed spectral decomposition.  Maybe that notation is standard now, but I had not seen it, and found it confusing, and had to work it out, as to what exactly they were doing.

Of course, I will be happy to consider specific reccomendations on various points of confusion if they are offered.  As the editors can appeciate, I do  however desire to keep the paper short.

Thanks for those comments Malcolm.  Yours on Abragam inspired me to look once again.  It is interesting that Bloch and Wagness and Bloch are cited, and he cites Hubbard's thesis.  Hubbard's review in Rev Mod Phys of course had just come out, so it could not be included.  And then so strange....Redfield's IBM article is not cited!  At least not in my 1961 edition.  Perhaps Abragam-Hubbard is one of those classic examples of "ships passing in the night", and one has to wonder how helpful Abragam would have found Hubbard's treatment.

Thanks for the update on Paul Hubbard.  In April 2020 I tried to track down Paul and send him a copy of my effort.  Charlie Johnson gave me his email.  I did not hear back from him on that, and of course I wondered about his health.   It would have been interesting to hear from him and I am sure he would have enjoyed learning about the new found interest in a topic related to his work of long ago.  This is a lesson I learned long ago, when I would always see I.I. Rabi having a coffe at the bar at Chock full of Nuts at 116th street.  And I often wanted to approach him and even invite him to come up to the lab were Ben Dailey and I were building a spectrometer, but I was too shy.  I always regreted that.

I will add one more comment about Paul Hubbard with interesting history.  When I got to Stony Brook, Charlie Springer approached me about double quantum coherences in isolated 23Na NMR in isotropic solutions.  Gerry Harbison was also there and both Gerry and I were doubtful because we were sure that you needed a bilinear term in the Hamiltonian etc etc. for double quantum effects.  But later that night, I recalled some parts of Hans Spiess's review article and I came in the next day and told Charlie that relaxation can produce these coherences and that I would work it all out if he was interested, and of course he was.  The reason Charlie was interested was that he was reviewing a paper by Jim Pekar and J.S. Leigh on this topic and had a long time interest in 23Na NMR.  When Charlie showed me that paper, I saw that the first reference was P.S. Hubbard JCP 53 985 (1970), and I thought "oh of course!".  Charlie, Bill Rooney and I wrote up a nice JACS paper on two quantum coherence in 23Na, but much to my chagrin, since I was the "theory guy", a similar effort was published prior in JCP by Wimperis and Bodenhausen.  Another interesting aspect was on the experimental side.  The spectrometer at Stony Brook was a venerable NT300 four phase system and since I had built a spectrometer at Columbia, I was always interested in multipulse tune up sequences.  With these we could test and trim the 90 degree phase shifts and obtain very clean separations.