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 A057221 #30 Mar 17 2025 07:24:44
%S A057221 2,6,30,162,654,714,1370,1662,1722,2810,77142,156254,432974,1092242,
%T A057221 1245230
%N A057221 Numbers k such that 2^k + 19 is prime.
%C A057221 a(14) > 5*10^5. - _Robert Price_, Aug 27 2015
%C A057221 All terms are even. - _Robert Israel_, Aug 28 2015
%C A057221 For numbers k in this sequence, the number 2^(k-1)*(2^k+19) has deficiency 20 (see A223607). - _M. F. Hasler_, Jul 18 2016
%H A057221 Henri Lifchitz and Renaud Lifchitz (Editors), <a href="http://www.primenumbers.net/prptop/searchform.php?form=2%5En%2B19">Search for 2^n+19</a>, PRP Top Records.
%F A057221 a(n) = 2*A253774(n). - _Joerg Arndt_, Aug 28 2015
%p A057221 select(n -> isprime(2^n+19), [seq(2*i,i=1..10000)]); # _Robert Israel_, Aug 28 2015
%t A057221 Do[ If[ PrimeQ[ 2^n + 19 ], Print[ n ] ], {n, 1, 15000} ]
%t A057221 Select[Range[10000], PrimeQ[2^# + 19] &] (* _Vincenzo Librandi_, Aug 28 2015 *)
%o A057221 (Magma) [n: n in [0..1000] | IsPrime(2^n+19)]; // _Vincenzo Librandi_, Aug 28 2015
%o A057221 (PARI) for(n=1,oo,ispseudoprime(2^n+19)&&print1(n",")) \\ _M. F. Hasler_, Jul 18 2016
%Y A057221 Cf. A223607, A253774.
%Y A057221 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), this sequence (2^k+19), A057201 (2^k+21), A057203 (2^k+23).
%K A057221 nonn,more,hard
%O A057221 1,1
%A A057221 _Robert G. Wilson v_, Sep 17 2000
%E A057221 a(11)-a(13) from _Robert Price_, Aug 27 2015
%E A057221 Edited by _M. F. Hasler_, Jul 18 2016
%E A057221 a(14)-a(15) found by Stefano Morozzi, added by _Elmo R. Oliveira_, Nov 19 2023