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A130139 Let f denote the map that replaces k with the concatenation of its proper divisors, written in increasing order, each divisor being written in base 10 in the normal way. Then a(n) = prime reached when starting at 2n+1 and iterating f. 5

%I

%S 1,3,5,7,3,11,13,1129,17,19,37,23,5,313,29,31,311,1129,37,313,41,43

%N Let f denote the map that replaces k with the concatenation of its proper divisors, written in increasing order, each divisor being written in base 10 in the normal way. Then a(n) = prime reached when starting at 2n+1 and iterating f.

%C If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1.

%e n = 7: 2n+1 = 15 = 3*5 -> 35 = 5*7 -> 57 = 3*19 -> 319 = 11*29 -> 1129, prime, so a(7) = 1129.

%Y Cf. A130140, A130141, A130142. A bisection of A120716.

%K base,more,nonn

%O 0,2

%A _N. J. A. Sloane_, Jul 30 2007

%E The value of a(22) is currently unknown.

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Last modified February 24 05:48 EST 2018. Contains 299597 sequences. (Running on oeis4.)