%I #20 Dec 26 2022 16:30:17

%S 3,23,223,22222223,22222222223,222222222222222222222222222222222223

%N Primes of the form 20*R_k + 3, where R_k is the repunit (A002275) of length k.

%C The next term (a(7)) has 95 digits. - _Harvey P. Dale_, Dec 26 2022

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22223.htm#prime">Prime numbers of the form 22...223</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>

%F a(n) = (20*10^A056656(n) + 7)/9 = (2*10^A096506(n) + 7)/9.

%t Select[Table[FromDigits[PadLeft[{3},n,2]],{n,40}],PrimeQ] (* _Harvey P. Dale_, Dec 26 2022 *)

%Y Cf. A002275, A056656 (corresponding k, and count of digits 2 in a(n)), A096506.

%K nonn

%O 1,1

%A _Rick L. Shepherd_, Mar 26 2004

%E Edited by _Ray Chandler_, Feb 27 2012