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A103109 Numbers n such that 9*10^n + 8*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1

%I #21 Jan 17 2019 13:44:07

%S 1,3,4,6,9,12,16,34,129,243,252,529,4885,6363,12951,13188,29931,34888,

%T 49516,67431,77781

%N Numbers n such that 9*10^n + 8*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (89*10^n-17)/9 is prime.

%C a(22) > 10^5. - _Robert Price_, Nov 14 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/98887.htm#prime">Prime numbers of the form 988...887</a>.

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

%F a(n) = A101018(n) + 1.

%t Do[ If[ PrimeQ[(89*10^n - 17)/9], Print[n]], {n, 0, 10000}]

%o (PARI) is(n)=isprime(9*10^n + 8*(10^n-1)/9 - 1) \\ _Anders Hellström_, Nov 14 2015

%Y Cf. A002275, A101018.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 19 2005

%E a(15)-a(18) from Kamada data by _Robert Price_, Dec 14 2010

%E a(19) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 17 2013

%E a(20)-a(21) from _Robert Price_, Nov 14 2015

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)