On a long proof On wikipedia there is a claim that the Abel–Ruffini theorem was nearly proved by Paolo Ruffini, and that his proof spanned over $500$ pages, is this really true? I don't really know much abstract algebra, and I know that the length of a paper will vary due to the size of the font, but what could possibly take $500$ pages to explain? Did he have to introduce a new subject part way through the paper or what? It also says Niels Henrik Abel published a proof that required just six pages, how can someone jump from $500$ pages to $6$?

Math can be very difficult.
There are famous long proofs, like that of the Feit-Thompson theorem, whose initial proof took 255 pages of very intrincate arguments, or the classification of quasi-thin simple groups done by Aschbacher and Smith in 1221 pages, or well, the whole classification of simple finite groups —which is estimated in the tens of thousands of pages— of which the result of Aschbacher and Smith is a very small part.
