primes=. ('fractan'f.) ((1 }. 2 ^. (#~ *./@:e.&2 0"1@:q:))@:)
'17/91 78/85 19/51 23/38 29/33 77/29 95/23 77/19 1/17 11/13 13/11 15/14 15/2 55/1' (<555555) primes 2
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71