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 A033843 #23 Jul 18 2025 04:22:09
%S A033843 0,1,2,3,5,7,10,17,24,36,62,107,177,290,505,860,1526,2679,4750,8535,
%T A033843 15500,27995,50638,92246,168617,309561,571313,1056281,1961080,3650557,
%U A033843 6810670,12739574,23878645,44849427,84384508,159082253,300424743,568237005,1076431099,2042054332,3879202049
%N A033843 Number of twin primes < 2^n.
%C A033843 For n=2 only the lower member of the pair [3,5] is < 2^n. - _Hugo Pfoertner_, Feb 07 2024
%H A033843 Amiram Eldar, <a href="/A033843/b033843.txt">Table of n, a(n) for n = 1..53</a>
%H A033843 <a href="/index/Pri#primepop">Index entries for sequences related to numbers of primes in various ranges</a>.
%t A033843 a[n_] := Count[Range[2^n], _?(And @@ PrimeQ[# + {-1, 1}] &)]; Array[a, 20] (* _Amiram Eldar_, Jul 18 2025 *)
%Y A033843 Cf. A007053, A007508.
%Y A033843 Partial sums of A095017.
%K A033843 nonn,hard
%O A033843 1,3
%A A033843 Vasiliy Danilov (danilovv(AT)usa.net), Jun 15 1998
%E A033843 a(38) from _Alex Ratushnyak_, Jun 07 2013
%E A033843 a(39)-a(41) from _Hugo Pfoertner_, Feb 07 2024