OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..100
EXAMPLE
3 and 5 are twin primes and 2*3+1=7 is prime, so 3 is the first of twin primes and is a Sophie Germain prime; it is the least such 1-digit prime, so a(1)=3.
11 and 13 are twin primes and 2*11+1=23 is prime, so 11 is the first of twin primes and is a Sophie Germain prime; it is the least such 2-digit prime, so a(2)=11.
MATHEMATICA
f[n_] := Block[{k = 10^(n - 1)}, While[ !PrimeQ[k] || !PrimeQ[k + 2] || !PrimeQ[2k + 1], k++ ]; k]; Array[f, 18] (* Robert G. Wilson v, May 13 2006 *)
lndp[n_]:=Module[{p=NextPrime[10^n]}, While[NoneTrue[p+{2, -2}, PrimeQ] || !PrimeQ[2p+1], p=NextPrime[p]]; p]; Array[ lndp, 20, 0] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Apr 06 2019 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Pierre CAMI, May 05 2006
EXTENSIONS
Edited by Robert G. Wilson v, May 13 2006
STATUS
approved