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 A120716 a(1)=1, a(p)=p if p is a prime. Otherwise, start with n and iterate the map (k -> concatenation of proper divisors of k) until we reach a prime q; then a(n) = q. If we never reach a prime, a(n) = -1. 10
 1, 2, 3, 2, 5, 23, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS N. J. A. Sloane et al., Notes on the attempt to find a(8) EXAMPLE 4 -> 2, prime, so a(4) = 2. 6 -> 2,3 -> 23, prime, so a(6) = 23. 8 -> 2.4 -> 24 -> 2.3.4.6.8.12 -> 2346812 -> 2.4.13.26.52.45131.90262.180524.586703.1173406 -> 2413265245131902621805245867031173406 -> ? (see link for the continuation) 9 -> 3, prime, so a(9) = 3. 21 -> 3,7 -> 37, prime, so a(21) = 37. CROSSREFS Cf. A120712, A120713, A037274, A130139-A130142. A130139 is a bisection. Sequence in context: A318956 A086507 A133568 * A084318 A084317 A037279 Adjacent sequences:  A120713 A120714 A120715 * A120717 A120718 A120719 KEYWORD nonn,base,more,hard AUTHOR N. J. A. Sloane, Jul 19 2007 EXTENSIONS a(8) is currently unknown. STATUS approved

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Last modified December 12 19:25 EST 2018. Contains 318081 sequences. (Running on oeis4.)