cp's OEIS Frontend

A098959 Numbers k such that 2*10^k + 6*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%I A098959 #35 Jul 13 2023 12:27:48
%S A098959 1,2,3,5,6,7,11,21,30,68,73,169,176,345,823,1021,1191,2073,2755,10717,
%T A098959 14673,16754,17606,81029,120851,167965,200408
%N A098959 Numbers k such that 2*10^k + 6*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A098959 Also numbers k such that (8*10^k - 11)/3 is prime.
%C A098959 a(28) > 3*10^5. - _Robert Price_, Jul 13 2023
%H A098959 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/26663.htm#prime">Prime numbers of the form 266...663</a>.
%H A098959 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A098959 a(n) = A101964(n) + 1.
%e A098959 For k = 1, 2, 3, 5, 6, 7, we get 23, 263, 2663, 266663, 2666663 and 26666663 which are primes.
%t A098959 Do[ If[ PrimeQ[(8*10^n - 11)/3], Print[n]], {n, 0, 10000}]
%Y A098959 Cf. A002275, A101964.
%K A098959 more,nonn
%O A098959 1,2
%A A098959 Julien Peter Benney (jpbenney(AT)ftml.net), Oct 21 2004
%E A098959 a(15), a(16) & a(17) from _Ray Chandler_, Nov 04 2004
%E A098959 a(18) & a(19) from _Robert G. Wilson v_, Dec 17 2004
%E A098959 a(20)-a(23) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E A098959 a(24) from Kamada data by _Robert Price_, Jan 17 2015
%E A098959 a(25)-a(27) from _Robert Price_, Jul 13 2023