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Again, J's primes primitive trivializes the problem. can be computed quickly using 5 p: y, so it is sufficient to brute-force.

   (% 5&p:) i.9   NB. hook to compute n/φ(n)
0 1 2 1.5 2 1.25 3 1.16667 2
   (i. >./) (% 5&p:) i.9   NB. add second hook to find index of the max value
6                          NB. since the input is a list of indices, no other work is needed
   (i. >./) (% 5&p:) i. 1e6   NB. the answer, this takes roughly half a second on my Dell Precision 5530