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.

Comment from Klaus Brockhaus (klaus-brockhaus(AT)t-online.de), Aug 01 2007:

"The sequence, starting from the beginning but writing 0 when a number with more than 90 digits is reached, is:

1,3,5,7,3,11,13,53,17,19,73,23,5,313,29,31,113,0,37,197,41,43,0,47,7,173,53,0,193,

59,61,0,0,67,233,71,73,0,391393,79,9313991471335749211973,83,0,293,89,137,313,

0,97,0,101,103,0,107,109,373,113,0,391393,593,11,1993,0,127,433,131,197,0,137,139,

0,0,0,0,149,151,511793,0,157,1141931201,6113,163,0,167,13,0,173,0,593,179,181,

613,12575251553,0,0,191,193,0,197,199,673,..."

Contribution from Andrew Carter (acarter09(AT)newarka.edu), Dec 16 2008: (Start)

If this sequence were to do all terms, all even terms except 2 and 4 would be -1:

a simple proof is that an even number's smallest proper divisor is 2,

all even numbers except 2 and 4 have factors in front so the resulting number would be 10n+2 and n is an integer greater than 1,

and therefore an even number endng in 2 other than two - rinse and repeat (End)

n = 13: 2n+1 = 27 has proper divisors 3 and 9, so we get 93, which has proper divisors 3 and 31, so we get 313, prime. So a(13) = 313.

Cf. A130139, A130140, A130142, A120716.

Sequence in context: A076565 A100029 A099984 this_sequence A130142 A130139 A101088

Adjacent sequences: A130138 A130139 A130140 this_sequence A130142 A130143 A130144

base,more,nonn

Carsten Lund (lund(AT)research.att.com), Jul 30 2007, Aug 01 2007

The value of a(17) is currently unknown.