A076850 Numbers k such that (10^k + 2)/6 is prime.

1, 2, 3, 4, 6, 10, 12, 15, 33, 55, 56, 61, 154, 201, 462, 570, 841, 848, 1297, 1357, 2008, 2628, 2848, 3111, 6877, 9162, 17766, 33556

Offset: 1

Author

Ed Pegg Jr, Nov 23 2002

Keywords

• nonn
• more

Comments

Some of the larger entries may only correspond to probable primes.

a(26) > 8000. - Jon E. Schoenfield, Dec 24 2017

Programs

Magma

[n: n in [0..200] |IsPrime((10^n+2) div 6)]; // Vincenzo Librandi, Sep 24 2016

Mathematica

Select[Range[5000], PrimeQ[(10^# + 2) / 6] &] (* Vincenzo Librandi, Sep 24 2016 *)

PARI

isok(n) = isprime((10^n + 2)/6); \\ Michel Marcus, Aug 28 2013

Python

from sympy import isprime, prime, nextprime
def afind(limit, startk=1):
    k, tenk = startk, 10**startk
    for k in range(startk, limit+1):
        if isprime((tenk + 2)//6):
            print(k, end=", ")
        tenk *= 10
afind(1000) # Michael S. Branicky, Feb 26 2022

Extensions

More terms from Jason Earls, Nov 24 2002

a(25) from Jon E. Schoenfield, Dec 23 2017

a(26) from Michael S. Branicky, Feb 27 2022

a(27) from Michael S. Branicky, Mar 29 2023

a(28) from Michael S. Branicky, Aug 05 2024