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 A156790 #9 Jun 02 2025 01:23:07 %S A156790 0,1,8,41,183,770,3149,12730,51209,205356,822500,3292134,13172634, %T A156790 52698912,210812207,843281848,3373193506,13492906143,53971888157, %U A156790 215888078393,863553363881,3454215553470,13816866413106,55267474046659 %N A156790 Number of first quadrant lattice squares inside the circle x^2+y^2=(2^n)^2. %C A156790 a(n)/4^n converges to Pi/4 from below. %H A156790 Wikipedia, <a href="http://en.wikipedia.org/wiki/Gauss_circle_problem">Gauss circle problem</a> [From _Jaume Oliver Lafont_, Apr 20 2010] %e A156790 Let + represent a square inside the circle and x a square traversed by the circle. %e A156790 xx %e A156790 +x a(1)=1 %e A156790 xxx %e A156790 ++xx %e A156790 +++x %e A156790 +++x a(2)=8 %o A156790 (PARI) a(n)=sum(m=1,2^n-1,floor(sqrt(4^n-m^2))) %Y A156790 Cf. A057655. %Y A156790 Cf. A177144. [From _Jaume Oliver Lafont_, May 03 2010] %K A156790 nonn %O A156790 0,3 %A A156790 _Jaume Oliver Lafont_, Feb 15 2009 %E A156790 a(19) corrected by Sophia Keith, Sep 15 2024