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%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
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