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A329941
Least prime, p, such that 2*p*3^n - 1 and 2*p*3^n + 1 are twin primes.
0
2, 11, 2, 5, 43, 29, 53, 311, 113, 109, 367, 859, 647, 11, 2, 619, 13, 1051, 157, 2801, 3767, 5, 337, 1721, 3517, 41, 4013, 1879, 1873, 13649, 4637, 2909, 8387, 6521, 1453, 6599, 1277, 4801, 167, 1031, 11213, 4129, 4933, 199, 1427, 859, 9227, 5581, 863, 11959, 10453
OFFSET
1,1
EXAMPLE
2*2*3^1 - 1 = 11; 11 and 13 are twin primes so a(1)=2.
2*11*3^2 - 1 = 197; 197 and 199 are twin primes so a(2)=11 as no other prime p < 11 gives twin primes.
MATHEMATICA
Array[Block[{p = 2}, While[! AllTrue[2 p 3^# + {-1, 1}, PrimeQ], p = NextPrime@ p]; p] &, 51] (* Michael De Vlieger, Dec 24 2019 *)
PROG
(PARI) a(n) = {my(p=2); while (!isprime(2*p*3^n - 1) || !isprime(2*p*3^n + 1), p = nextprime(p+1)); p; } \\ Michel Marcus, Nov 25 2019
CROSSREFS
Cf. A130327.
Sequence in context: A298444 A224480 A037299 * A347121 A341512 A276676
KEYWORD
nonn
AUTHOR
Pierre CAMI, Nov 24 2019
STATUS
approved