cp's OEIS Frontend

A130140 Let f denote the map that replaces k with the concatenation of its nontrivial divisors, written in increasing order, each divisor being written in base 10 with its digits in reverse order. Then a(n) = prime reached when starting at 2n+1 and iterating f.

%I A130140 #13 Mar 09 2023 08:40:41
%S A130140 1,3,5,7,3,11,13
%N A130140 Let f denote the map that replaces k with the concatenation of its nontrivial divisors, written in increasing order, each divisor being written in base 10 with its digits in reverse order. Then a(n) = prime reached when starting at 2n+1 and iterating f.
%C A130140 If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1.
%e A130140 n = 7: 2n+1 = 15 = 3*5 -> 35 = 5*7 -> 57 = 3*19 -> 391 = 17*23 -> 7132.
%e A130140 Then 7132 has nontrivial divisors 2, 4, 1783, 3566, so we get 2438716653.
%e A130140 Then 2438716653 has nontrivial divisors 3, 9, 27, 81, 243, 1453, 4359, 6907, 13077, 20721, 39231, 62163, 117693, 186489, 353079, 559467, 1678401, 10035871, 30107613, 90322839, 270968517, 812905551, so we get
%e A130140 397218342354195347096770311270213293361263967119846819703537649551048761178530013167010393822309715869072155509218 = 2*3^4*1217*317539*1211548321*33378971294653*8960783431807*17509226460292689821646170308388500174366980857582533580184934929433.
%Y A130140 Cf. A130139, A130141, A130142, A120716.
%K A130140 base,more,nonn
%O A130140 0,2
%A A130140 _David Applegate_, Jul 30 2007
%E A130140 Edited by _Michel Marcus_, Mar 09 2023