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A068814 Primes with a 2 followed by (possibly zero) 1's. 1
2, 211, 2111, 2111111111111, 2111111111111111111, 211111111111111111111111, 2111111111111111111111111111111111111111111111111111111111, 211111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111 (list; graph; refs; listen; history; text; internal format)
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

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Last modified November 20 13:03 EST 2019. Contains 329336 sequences. (Running on oeis4.)