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 A086680 #21 Jun 06 2024 06:23:01 %S A086680 0,2,6,18,54,172,564,1900,6542,23000,82025,295947,1077871,3957809, %T A086680 14630843,54400028,203280221,762939111,2874398515,10866266172, %U A086680 41203088796,156661034233,597116381732,2280998753949,8731188863470,33483379603407,128625503610475,494890204904784 %N A086680 Number of primes < 4^n. %C A086680 These numbers approximate the sum of primes < 2^n. The sum of primes < 2^20 = A130739(20) = 41162256126. The number of primes < 2^40 = 41203088796. The error is -0.000991993. From A007053, we have number of primes < 2^74 = 375744164937699609596 and sum of primes < 2^37 = 375743480734175191581 for an error of -0.0000018209325. %H A086680 Amiram Eldar, <a href="/A086680/b086680.txt">Table of n, a(n) for n = 0..46</a> (calculated from the b-file at A007053) %H A086680 Cino Hilliard, <a href="http://groups.google.com/group/primes-less-than-x/web/sum-of-primes-less-than-n">Sum of primes less than n</a> [Dead link] %F A086680 a(n) = A007053(2*n). - _Amiram Eldar_, Jun 06 2024 %e A086680 a(2) = 6 because there are 6 primes, 2, 3, 5, 7, 11, and 13 that are < 4^2. %Y A086680 Cf. A007053, A130739. %K A086680 nonn %O A086680 0,2 %A A086680 _Cino Hilliard_, Jul 28 2003 %E A086680 Extended by _Cino Hilliard_, Feb 13 2009 %E A086680 a(25)-a(27) from _Amiram Eldar_, Jun 06 2024