A056721 - OEIS

%I #35 Jun 18 2025 11:24:04

%S 0,1,4,5,8,10,25,49,76,128,175,238,550,796,1219,2012,2846,11336,21296,

%T 49808,74318,236891,608989,1652593,1715905,1990324,2045966,2084563,

%U 5112847

%N Numbers k such that 8*10^k-1 is prime.

%C Also numbers n such that 7*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79999.htm#prime">Prime numbers of the form 799...99</a>.

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

%t Do[ If[ PrimeQ[ 7*10^n + (10^n-1)], Print[n]], {n, 0, 3000}]

%o (PARI) is(n)=ispseudoprime(8*10^n-1) \\ _Charles R Greathouse IV_, Feb 20 2017

%K nonn,hard,more

%O 1,3

%A _Robert G. Wilson v_, Aug 11 2000

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(22)-a(23) from Kamada data by _Robert Price_, Oct 19 2014

%E a(24)-a(25) from Kamada data by _Tyler Busby_, Apr 15 2024

%E Missing terms from _Ryan Propper_, Jun 02 2025