A299383 - OEIS

%I #10 Sep 08 2022 08:46:20

%S 1,18,44,60,80,123,429,1166,2065,8774,35340,42968,50312,210129

%N Numbers k such that k * 20^k - 1 is prime.

%C a(15) > 400000.

%H Steven Harvey, <a href="http://harvey563.tripod.com/GeneralizedWoodallPrimes.txt">Generalized Woodall Search</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Woodall_number">Woodall number</a>

%t Select[Range[1, 10000], PrimeQ[n*20^n-1] &]

%o (PARI) for(n=1, 10000, if(isprime(n*20^n-1)), print1(n, ", ")))

%o (Magma) [n: n in [1..10000] |IsPrime(n*20^n-1)]

%Y Numbers n such that n * b^n - 1 is prime: A008864 (b=1), A002234 (b=2), A006553 (b=3), A086661 (b=4), A059676 (b=5), A059675 (b=6), A242200 (b=7), A242201 (b=8), A242202 (b=9), A059671 (b=10), A299374 (b=11), A299375 (b=12), A299376 (b=13), A299377 (b=14), A299378 (b=15), A299379 (b=16), A299380 (b=17), A299381 (b=18), A299382 (b=19), this sequence (b=20).

%K nonn,more,hard

%O 1,2

%A _Tim Johannes Ohrtmann_, Feb 08 2018