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.

1, 3, 5, 7, 3, 11, 13

Offset: 0

Author

David Applegate, Jul 30 2007

Keywords

• base
• more
• nonn

Comments

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

Examples

			n = 7: 2n+1 = 15 = 3*5 -> 35 = 5*7 -> 57 = 3*19 -> 391 = 17*23 -> 7132.
Then 7132 has nontrivial divisors 2, 4, 1783, 3566, so we get 2438716653.
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
397218342354195347096770311270213293361263967119846819703537649551048761178530013167010393822309715869072155509218 = 2*3^4*1217*317539*1211548321*33378971294653*8960783431807*17509226460292689821646170308388500174366980857582533580184934929433.
		

Crossrefs

Cf. A130139, A130141, A130142, A120716.

Extensions

Edited by Michel Marcus, Mar 09 2023