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 A185192 #36 Jul 07 2024 18:11:34 %S A185192 0,2,4,6,11,18,31,54,97,172,309,564,1028,1900,3512,6542,12251,23000, %T A185192 43390,82025,155611,295947,564163,1077871,2063689,3957809,7603553, %U A185192 14630843,28192750,54400028,105097565,203280221,393615806,762939111,1480206279,2874398515 %N A185192 Number of primes less than 2^n. %C A185192 This sequence differs from A007053 in the offset and in that the latter includes 1 (since the prime 2 is equal to 2^1, not less than it). %C A185192 Number of primes with at most n bits. %H A185192 David Baugh, <a href="/A185192/b185192.txt">Table of n, a(n) for n = 1..90</a> (terms n = 1..81 from Charles R Greathouse IV). %H A185192 Wikipedia, <a href="http://en.wikipedia.org/wiki/Prime-counting_function">Prime-counting function</a>. %F A185192 a(n) = A000720(2^n - 1). %F A185192 a(n) = A007053(n) for n > 1. %e A185192 a(7) = 31 because prime(31) = 127, prime(32) = 131, and 127 < 2^7 < 131. %t A185192 PrimePi[2^Range[32] - 1] (* _Alonso del Arte_, May 05 2019 *) %o A185192 (PARI) a(n)=primepi(2^n-1) \\ _Charles R Greathouse IV_, Dec 04 2014 %Y A185192 Cf. A000720, A006880, A007053, A014234. %K A185192 nonn,hard %O A185192 1,2 %A A185192 _Washington Bomfim_, Jan 23 2012