0,2

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

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.

Cf. A130139, A130141, A130142, A120716.

Sequence in context: A131214 A104260 A121573 this_sequence A051417 A090368 A120374

Adjacent sequences: A130137 A130138 A130139 this_sequence A130141 A130142 A130143

base,more

David Applegate (david(AT)research.att.com), Jul 30 2007