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 A130140 Let f denote the map that replaces k by the concatenation of its proper 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. 4
 1, 3, 5, 7, 3, 11, 13 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1. LINKS EXAMPLE n = 7: 2n+1 = 15 = 3*5 -> 35 = 5*7 -> 57 = 3*19 -> 391 = 17*23 -> 7132. Then 7132 has proper divisors 2, 4, 1783, 3566, so we get 2438716653. Then 2438716653 has proper 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, which has a lot of divisors. So a(7) is currently unknown. CROSSREFS Cf. A130139, A130141, A130142, A120716. Sequence in context: A121573 A196407 A156030 * A051417 A090368 A120374 Adjacent sequences:  A130137 A130138 A130139 * A130141 A130142 A130143 KEYWORD base,more,nonn AUTHOR David Applegate, Jul 30 2007 STATUS approved

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