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%I #16 Jan 17 2019 13:44:07
%S 0,2,3,86,134,185,344,396,476,834,1799,2147,2418,5216,5882,6216,13394,
%T 19746,66485
%N Numbers n such that 5*10^n + 4*R_n - 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (49*10^n-31)/9 is prime.
%C a(20) > 10^5. - _Robert Price_, Jul 16 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/54441.htm#prime">Prime numbers of the form 544...441</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101578(n-1) + 1, for n>1.
%t Do[ If[ PrimeQ[(49*10^n - 31)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101578.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E a(17)-a(18) from Kamada data by _Robert Price_, Dec 14 2010
%E a(1) = 0 prepended by _Robert Price_, Jul 16 2015
%E a(19) from _Robert Price_, Jul 16 2015
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