A056717 - OEIS

%I #28 Jul 08 2021 00:45:54

%S 1,5,7,25,31,112,199,533,616,718,787,1357,2779,3889,4192,7537,7945,

%T 23938,32632,49169,56453,61097,90211

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

%C Also numbers k such that (55*10^k - 1)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/6/61111.htm#prime">Prime numbers of the form 611...11</a>.

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

%t Do[ If[ PrimeQ[ 6*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A093631.

%K nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 11 2000

%E 2779, 3889, 4192, 7537, 7945 from _Hugo Pfoertner_, Oct 19 2004

%E 23938 from _Ray Chandler_, Sep 25 2010

%E 32632 and 49169 from _Ray Chandler_, Feb 08 2012

%E 56453 from Serge Batalov, Mar 12 2009 confirmed as next term by _Ray Chandler_, Feb 09 2012

%E 61097 from _Ray Chandler_, Feb 09 2012

%E 90211 from _Ray Chandler_, Feb 10 2012