OFFSET
1,2
COMMENTS
Conjecture: a(n) < prime(n) except for a(522720).
a(522720)=12582912 > p(522720)=7728803.
Conjecture tested up to a(1000000).
LINKS
Lei Zhou, Table of n, a(n) for n = 1..10000
EXAMPLE
prime(1) = 2, 2 * 1 + 1 = 3 is prime, so a(1)=1;
prime(2) = 3, 3 * 2 + 1 = 7 is prime, so a(2)=2;
......
prime(7) = 17, 17 * 1 + 1 = 18 is not prime,
17 * 2 + 1 = 35 is not prime,
17 * 3 + 1 = 52 is not prime,
17 * 4 + 1 = 69 is not prime,
17 * 6 + 1 = 103 is prime, so a(7)=6
MATHEMATICA
f[n_] := Block[{p2, p3 = 3^Range[0, Floor@ Log[3, n] + 1]}, p2 = 2^Floor[Log[2, n/p3] + 1]; Min[ Select[ p2*p3, IntegerQ]]]; Table[pr=Prime[i]; j=1; fj=0; While[j++; fj=f[fj+1/2]; cp=1+pr*fj; !PrimeQ[cp]]; fj, {i, 115}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lei Zhou, Nov 19 2012
STATUS
approved