This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A057201 #27 Nov 27 2023 10:30:02
%S A057201 1,3,4,5,7,8,11,15,16,19,44,48,51,52,61,163,196,456,492,911,997,1616,
%T A057201 1631,1647,1803,1899,3112,3584,3956,6848,7023,9535,16657,27035,33843,
%U A057201 36551,38859,81485,107287,131383,139476,158497,210061,216752,339168,341355,376731,1173095
%N A057201 Numbers k such that 2^k + 21 is prime.
%C A057201 a(48) > 5*10^5. - _Robert Price_, Sep 17 2015
%H A057201 Henri Lifchitz and Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=2%5En%2B21">Search for 2^n+21</a>, PRP Top Records.
%e A057201 k = 15, 2^15 + 21 = 32789 is prime.
%e A057201 k = 16, 2^16 + 21 = 65557 is prime.
%t A057201 Do[ If[ PrimeQ[ 2^n + 21 ], Print[ n ] ], {n, 1, 4000} ]
%t A057201 Select[Range[10000], PrimeQ[2^# + 21] &] (* _Vincenzo Librandi_, Aug 28 2015 *)
%o A057201 (Magma) [n: n in [0..1000] | IsPrime(2^n+21)]; // _Vincenzo Librandi_, Aug 28 2015
%o A057201 (PARI) is(n)=isprime(2^n+21) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y A057201 Cf. A094076.
%Y A057201 Cf. A019434 (primes 2^k+1), A057732 (2^k+3), A059242 (2^k+5), A057195 (2^k+7), A057196 (2^k+9), A102633 (2^k+11), A102634 (2^k+13), A057197 (2^k+15), A057200 (2^k+17), A057221 (2^k+19), this sequence (2^k+21), A057203 (2^k+23).
%K A057201 nonn
%O A057201 1,2
%A A057201 _Robert G. Wilson v_, Sep 16 2000
%E A057201 a(30)-a(47) from _Robert Price_, Dec 06 2013
%E A057201 a(48) found by Stefano Morozzi, added by _Elmo R. Oliveira_, Nov 25 2023