Synonyms for "old" with positive connotations (and some conditions)

I want to describe a project for categorizing 'old' things, as well as antedating things that may not be very old*. I'm looking for a word to describe their age.

First, I would like a positive or at least neutral connotation. "Antedate" is good in this sense, "old" not so much. (But I don't want to restrict the scope to just things that are known at the present but for which older uses are found, but also to include categorizing existing old things and discovering previously-unknown old things.)

Second, the term should not imply great antiquity. The things under consideration are a type of mathematical object and most will be fairly recent (19th or 20th century), though a few will be older.

Finally the term should not be overly obscure. Perhaps there is a common word which I have overlooked; if not, at least it should be a word with some currency.

At the moment I'm leaning toward "historical" which is not great but may suffice. Can you find a better word?

* That is, for something known to date back to N, find a use of it from year M where M < N. "Fibonacci's identity was actually first studied by Diophantus 900 years earlier."

In this context, maybe a phrase, such as well-studied would be good. It implies that the mathematical objects have been known for a while, but does not have any negative connotations (to me), nor implies great antiquity. I think it depends a lot on the context. Had you asked for a similar word to describe cheese, I'd have suggested aged or well aged, but I don't think that applies at all in this case. ;)

I re-wrote one of your sentences to not use the word "old". In the other two contexts, I honestly don't see anything wrong with it.

The Foo sequence is named after John Foo, but it is actually the same sequence as earlier discovered by Amy Bar in 1945.

You may refer to the objects as classical.

The word we use in the computer industry is legacy. It can have the positive connotation of a bequest from past masters. Or, it can have the negative connotation of a sort of white elephant that one must take care of even though it is obsolete. The connotation is usually clear from context.