%I #14 Jan 17 2019 13:44:05
%S 2,211,2111,2111111111111,2111111111111111111,
%T 211111111111111111111111,
%U 2111111111111111111111111111111111111111111111111111111111,211111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
%N Primes with a 2 followed by (possibly zero) 1's.
%C Primes of the form (19*10^n-1)/9.
%C The next term (a(9)) has 544 digits. _Harvey P. Dale_, Sep 28 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/21111.htm#prime">Prime numbers of the form 211...11</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = (19*10^A056700(n)-1)/9.
%t Select[Table[FromDigits[Join[{2},PadRight[{},n,1]]],{n,0,130}],PrimeQ] (* _Harvey P. Dale_, Sep 28 2015 *)
%o (PARI) for(n=0,200, if(isprime(2*10^n+(10^(n)-1)/9)==1,print1(2*10^n+(10^(n)-1)/9,",")))
%Y Cf. A056700.
%K nonn
%O 1,1
%A _Amarnath Murthy_, Mar 07 2002
%E More terms from _Benoit Cloitre_, Mar 09 2002
%E Edited by _Ray Chandler_, Feb 03 2012