A102948 - OEIS

%I #18 Jul 08 2021 01:36:47

%S 1,3,7,13,24,30,48,52,163,175,219,228,261,754,951,1344,1573,3294,3523,

%T 14161,70794

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

%C Also numbers k such that (19*10^k + 17)/9 is prime.

%C a(22) > 10^5. - _Robert Price_, Mar 04 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/21113.htm#prime">Prime numbers of the form 211...113</a>.

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

%F a(n) = A101953(n) + 1.

%t Do[ If[ PrimeQ[(19*10^n + 17)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101953.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Dec 17 2004

%E Addition of a(20) from Kamada data by _Robert Price_, Dec 09 2010

%E a(21) from _Robert Price_, Mar 04 2015