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 A309751 #13 Jun 22 2022 10:38:18
%S A309751 0,0,2,2,10,10,10,74,202,202,714,714,714,714,8906,25290,58058,123594,
%T A309751 254666,516810,516810,1565386,1565386,5759690,14148298,14148298,
%U A309751 47702730,47702730,181920458,450355914,987226826,987226826,3134710474,7429677770,7429677770
%N A309751 Approximation of the 2-adic integer arctan(2) up to 2^n.
%C A309751 arctan(x) = x - x^3/3 + x^5/5 - x^7/7 + ...
%H A309751 Wikipedia, <a href="https://en.wikipedia.org/wiki/P-adic_number">p-adic number</a>
%F A309751 a(n) = (Sum_{i=0..floor(n/2)-1} (-1)^i*2^(2*i+1)/(2*i+1)) mod 2^n.
%e A309751 a(2) = 2^1 mod 2^2 = 2;
%e A309751 a(3) = 2^1 mod 2^3 = 2;
%e A309751 a(4) = (2^1 - 2^3/3) mod 2^4 = 2;
%e A309751 a(5) = (2^1 - 2^3/3) mod 2^5 = 10;
%e A309751 a(6) = (2^1 - 2^3/3 + 2^5/5) mod 2^6 = 10;
%e A309751 a(7) = (2^1 - 2^3/3 + 2^5/5) mod 2^7 = 74.
%o A309751 (PARI) a(n) = lift(sum(i=0, n/2-1, Mod((-1)^i*2^(2*i+1)/(2*i+1), 2^n)))
%Y A309751 Cf. A309752, A309753.
%K A309751 nonn
%O A309751 0,3
%A A309751 _Jianing Song_, Aug 15 2019
%E A309751 Offset corrected by _Georg Fischer_, Jun 22 2022