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

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

Offset

1,1

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

Table of n, a(n) for n=1..8.

Makoto Kamada, Prime numbers of the form 211...11.

Index entries for primes involving repunits

Formula

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

Mathematica

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

Prog

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

Crossrefs

Cf. A056700.

Sequence in context: A106100 A167839 A107612 * A215641 A195335 A090560

Adjacent sequences: A068811 A068812 A068813 * A068815 A068816 A068817

Keyword

nonn

Author

Amarnath Murthy, Mar 07 2002

Extensions

More terms from Benoit Cloitre, Mar 09 2002

Edited by Ray Chandler, Feb 03 2012

Status

approved