CFSG, checked

The Classification of Finite Simple Groups, or CFSG, was undoubtedly the major collective achievement of mathematics in our time. Daniel Gorenstein, the general who had masterminded the campaign, announced 45 years ago that the proof was finished apart from some details which could be done later (proving that groups of Ree type were Ree groups, and the uniqueness of the fourth Janko group, for example). It turned out that the announcement was premature, since a big component, the classification of quasi-thin groups, had not been done. (Rumour said that Geoff Mason had written a long manuscript handling this case, but that was not so.) It took another quarter of a century until Michael Aschbacher and Steve Smith stepped up and dealt with the quasi-thin groups. (In fact, the definition of this class had slightly changed in the meantime, because of the way it fitted into the general proof.) The entire proof was estimated to be about 15000 pages long, scattered over many books and journal…