Here's what András Sárközy, Erdős's most prolific collaborator, asked 25 years ago:
"How small can one make the maximal gap between the consecutive elements of a multiplicative Sidon set selected from {1, 2, ..., n}?"
In particular: does there exist a multiplicative Sidon set A ⊆ {1, 2, …, n} such that every sub-interval of [1, n] of length at least √n contains at least one element of A?
The answer is yes. The solution was autonomously discovered and formally verified in
#Lean by Aristotle. We then improved the bound below √n and Aristotle formalised our proof too.