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 A105603 #9 Feb 16 2025 08:32:57
%S A105603 1,1,3,5,11,7,43,17,57,31,683,13,2731,127,331,257,43691,73,174763,205,
%T A105603 5419,2047,2796203,241,1016801,8191,261633,3277,178956971,151,
%U A105603 715827883,65537,1397419,131071,24214051,4033,45812984491,524287,22366891,61681
%N A105603 Sylvester-Jacobsthal cyclotomic numbers.
%C A105603 Primitive parts of Jacobsthal numbers A001045.
%H A105603 T. D. Noe, <a href="/A105603/b105603.txt">Table of n, a(n) for n=1..1000</a>
%H A105603 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SylvesterCyclotomicNumber.html">Sylvester Cyclotomic Number</a>.
%F A105603 a(n)=product{k=1..n-1, if(gcd(n, k)=1, 2+exp(2*pi*I*k/n), 1)}, I=sqrt(-1)
%F A105603 a(n) = (-1)^phi(n)*cyclotomic(n, -2) for n >= 2 (for n = 1 this would be 3), with phi(n) = A000010(n). - _Wolfdieter Lang_, Jan 19 2015
%Y A105603 Cf. A020501, A001045, A061446 (with references), A008555.
%K A105603 easy,nonn
%O A105603 1,3
%A A105603 _Paul Barry_, Apr 15 2005