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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

%I

%S 1,2,3,2,5,23,7

%N 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.

%H N. J. A. Sloane et al., <a href="/A120716/a120716.txt">Notes on the attempt to find a(8)</a>

%e 4 -> 2, prime, so a(4) = 2.

%e 6 -> 2,3 -> 23, prime, so a(6) = 23.

%e 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)

%e 9 -> 3, prime, so a(9) = 3.

%e 21 -> 3,7 -> 37, prime, so a(21) = 37.

%Y Cf. A120712, A120713, A037274, A130139-A130142. A130139 is a bisection.

%K nonn,base,more,hard

%O 1,2

%A _N. J. A. Sloane_, Jul 19 2007

%E a(8) is currently unknown.

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Last modified January 17 23:15 EST 2019. Contains 319251 sequences. (Running on oeis4.)