A091189 - OEIS

%I #12 Jul 08 2021 06:23:54

%S 2221,222222222222222221,

%T 2222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222221

%N Primes of the form 20*R_k + 1, where R_k is the repunit (A002275) of length k.

%C Primes of the form 222...221.

%C The number of 2's in each term is given by the corresponding term of A056660 and so the first term too large to include above is 222...2221 (with 120 2's).

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22221.htm#prime">Prime numbers of the form 22...221</a>.

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

%Y Cf. A056660 (corresponding k), A084832.

%Y Cf. A051200, A004022, A093176, A093177, A089346.

%K bref,nonn

%O 1,1

%A _Rick L. Shepherd_, Feb 22 2004