The background: In the grand area of Mathematics in which I do research, there is a famous Professor, let us call him 'A', who started a joint project with another younger colleague, let us call him 'B'. 'A' gave the main ideas and they produced an initial draft. However, after a while 'A' perhaps lost interest or was busy with other things and stopped communicating with 'B'. 'B' could not manage to move the project forward by himself, and it stayed put. After many years, on the initiative of a third colleague, let us call him say 'C', and with both the approval of 'A' and 'B', I joined them on this project and started working with 'B'. After, say, two years, 'B' and I managed to have a paper, which we think is quite good. We have sent it to 'A' to hear his opinion. However, he never returns our email messages. We have also contacted his secretary, and she has been very helpful, but we don't seem to get any reply from 'A'. Because of the pandemic, travelling and meeting 'A' in person is not an option.
'B' and I would like to distribute and eventually publish this work. We have spent quite some time and energy on it. Moreover, the results are interesting and could be used by ourselves in other projects, as well as by other researchers. If we wait too much, there is even the risk of our paper becoming old and superseded by other people's work.
Of course we cannot post this work publically without 'A's name, as he was who gave the key ideas. On the other hand, posting this work with his name could also upset 'A', say, he could think there are mistakes in the paper and he would not want to have his name associated with it.
What is the ethical way to solve this?
Edit: Thanks for all answers! They really add to the discussion. In view of your comments, I feel like adding some more details to the story and making more clear our position.
It is not that A just gave some vague ideas. He really contributed technical stuff during the first year or so of his collaboration with B. What I did with B, in the past two years, was cleaning up the Mathematics and adding some interesting applications. I think we have really turned the draft into a (quasi?) paper. I believe our manuscript is polished.
A is well known for his irresponsiveness in the community of researchers in the field. Indeed this is the reason why there has been the hiatus in the work on the paper, until I joined forces with B.
B and I met A in conferences in the past two years. He was nice and seemed interested in finishing the work. However, on those occasions there was little time to collaborate with him, as he is a busy person.
B and I are fine with whatever course of action A thinks it is appropriate. We do not require him to check our calculations. We are fine to include him as an author, indeed we would like very much to have him as a co-author. We are fine to leave his name out, if he prefers so. But I have the impression he wants to be included.
Oh, yes, we certainly want to keep a collegial relation with A!
Anyway, we cannot proceed without his response!