In the case the referent is specific. and so if an article is used it should be definite. But
The Hilbert twenty-fourth problem
is incorrect. (as FumbleFingers says in a comment, this is because a "number" adjective should come before the noun in normal use.) It could be rephrased as
- The twenty-fourth Hilbert problem
- Hilbert's twenty-fourth problem
However, a definite article should not be used with the possessive form:
- The Hilbert's twenty-fourth problem
That seems to apply the article to "Hilbert", as would be correct in a construction like:
The President's twenty-fourth appointment
But we do not speak of 'The Hilbert" so this form is incorrect and will sound very odd to a fluent speaker.
However, when a referenced term is not specific, an indefinite article should be used:
{X} is a famous proof by Galois
Here X is one of many famous proofs, so an indefinite article would be used. if it was a proof by a mathematician known for only one result, a definite article could be used:
{X} is the famous proof by Jones.
But an indefinite article could also be used, because although there is only one famous proof by Jones, there are many famous proofs by one person or another. This is a matter of emphasis, are we focusing on Jones, or on the proof as one of many famous proofs. However, in the case of:
{X} is a famous proof.
Only an indefinite article is appropriate, because ther are many famous proofs, and X is in no way specific among them (or if it is we have not said so).
In short it is not that there is reference to a thing defined elsewhere that determines what sort of article to use, it is the meaning and structure of the text itself, and particularly whether the thing is being discussed in a specific or general way.