A056712 - OEIS

%I #24 Sep 08 2022 08:45:01

%S 2,3,4,6,14,54,210,390,594,3460,5028,5219,5332,8072,15796,16131,21456,

%T 29282,78790,85142,179972,211029,445773,464843,511056

%N Numbers n such that 5*10^n-1 is prime.

%C Also numbers n such that 4*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/4/49999.htm#prime">Prime numbers of the form 499...99</a>.

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

%t Do[ If[ PrimeQ[ 4*10^n + (10^n-1)], Print[n]], {n, 0, 4660}]

%o (Magma) [n: n in [0..2000] |IsPrime(5*10^n-1)]; // _Vincenzo Librandi_, Oct 20 2014

%o (PARI) is(n)=ispseudoprime(5*10^n-1) \\ _Charles R Greathouse IV_, Mar 01 2017

%K hard,nonn,more

%O 1,1

%A _Robert G. Wilson v_, Aug 11 2000

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

%E a(21)-a(25) from Kamada data by _Robert Price_, Oct 19 2014