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%I #21 Jan 21 2022 03:44:08
%S 3,6207,8076,22356,151456
%N Numbers k such that k * 20^k + 1 is prime.
%C a(6) > 219976.
%H Steven Harvey, <a href="http://harvey563.tripod.com/GClist.txt">Generalized Cullen Search</a>
%H G. Loeh, <a href="http://guenter.loeh.name/gc/status.html">Generalized Cullen primes</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Cullen_number">Cullen 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 k such that k * b^k + 1 is prime: A006093 (b=1), A005849 (b=2), A006552 (b=3), A007646 (b=4), A242176 (b=6), A242177 (b=7), A242178 (b=8), A265013 (b=9), A007647(b=10), A242196(b=12), A242197 (b=14), A242198 (b=15), A242199 (b=16), A007648 (b=18), this sequence (b=20).
%K nonn,more,hard
%O 1,1
%A _Tim Johannes Ohrtmann_, Oct 25 2020
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