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 A040018 #19 Mar 06 2021 02:11:17
%S A040018 1,1,3,8,46,176,955,5446,36122,259084,2043944,17371318,159616680,
%T A040018 1566391958
%N A040018 (Number of permutations of {1,2,...,n} for which sums of adjacent numbers are all distinct)/2n.
%C A040018 Essentially different ways 1,2,3,...,n can be placed around a circle so that sums of adjacent numbers are distinct.
%H A040018 Sean A. Irvine, <a href="https://github.com/archmageirvine/joeis/blob/master/src/irvine/oeis/a040/A040018.java">Java program</a> (github)
%Y A040018 Cf. A091217, A006967.
%K A040018 nonn,hard,more
%O A040018 3,3
%A A040018 _Erich Friedman_
%E A040018 a(10) from _Vladeta Jovovic_, Aug 25 2007
%E A040018 a(11)-a(13) from _Steve Butler_, Feb 08 2012
%E A040018 a(14)-a(16) from _Sean A. Irvine_, Mar 05 2021