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 A175955 #13 Mar 13 2019 05:20:54 %S A175955 1,0,1,1,4,6,18,36,92,209,527,1269,3218,8063,20701,53209,138634, %T A175955 362789,957857,2541735,6787960,18214250,49120018,133024306,361736098, %U A175955 987284765,2703991469,7429359867,20473889132,56579399002,156766505690 %N A175955 Number of ways to connect with nonintersecting chords n unlabeled points equally spaced on a circle such that the resulting configuration is not invariant w.r.t. rotation any angle < 2*Pi. %C A175955 Also, number of chord configurations on n vertices of the period n. %C A175955 Number of such chord configurations on 2n vertices with n chords is given by A005354(n+1). %F A175955 For odd prime p, a(p) = (A001006(p)-1)/p = A175954(p)-1. %e A175955 For n=2, there are only two configurations possible: two diametrically located points on a circle connected or not connected with a chord. Since both these configurations are invariant w.r.t. rotation by angle Pi, a(2)=0. %Y A175955 Cf. A001006, A175954. %K A175955 nonn %O A175955 1,5 %A A175955 _Max Alekseyev_, Oct 29 2010