A068814 Primes with a 2 followed by (possibly zero) 1's.

2, 211, 2111, 2111111111111, 2111111111111111111, 211111111111111111111111, 2111111111111111111111111111111111111111111111111111111111, 211111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111

Offset: 1

Author

Amarnath Murthy, Mar 07 2002

Keywords

• nonn

Comments

Primes of the form (19*10^n-1)/9.

The next term (a(9)) has 544 digits. Harvey P. Dale, Sep 28 2015

Links

• Makoto Kamada, Prime numbers of the form 211...11.
• Index entries for primes involving repunits

Crossrefs

Cf. A056700.

Programs

Mathematica

Select[Table[FromDigits[Join[{2},PadRight[{},n,1]]],{n,0,130}],PrimeQ] (* Harvey P. Dale, Sep 28 2015 *)

PARI

for(n=0,200, if(isprime(2*10^n+(10^(n)-1)/9)==1,print1(2*10^n+(10^(n)-1)/9,",")))

Formula

a(n) = (19*10^A056700(n)-1)/9.

Extensions

More terms from Benoit Cloitre, Mar 09 2002

Edited by Ray Chandler, Feb 03 2012