The questions above use the term 'lost' to refer to theorems that exist in the literature, but may be so obscure and/or not commonly useful nowadays that few people actually bother to learn them. Are there any theorems that were truly lost - that is, they were reasonably well-received by the mathematical community, were used for some time, and then were truly lost to history, unavailable in any extant literature, textbook, or other resource. All available copies might have been lost due to fire, war, or an intentional purge, or available copies might remain but only in an undeciphered code. Examples would include:
- Theorems that have been truly forgotten and literally cannot be learned today from any source
- Theorems that were truly lost but were rediscovered many years later, after the original author and all of their students were deceased
I am not talking about theorems that are commonly classified as lost simply due to obscurity or uselessness, but can be learned by a student determined enough to see out applicable resources, instructors, etc. These are what the two questions at the top are speaking of.
To some extent, Fermat's Last Theorem seems to match this, but I understand that there is quite a bit of controversy whether Fermat ever even had a proof to begin with.