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 A321415 #10 Nov 09 2018 21:33:37 %S A321415 0,2,2,10,2,64,2,330,1028,2010,2,24216,2,77528,964696,490314,2, %T A321415 11437750,2,21390330 %N A321415 Number of n element multisets of the 2n-th roots of unity with zero sum. %C A321415 Equivalently, the number of closed convex paths of length n whose steps are the 2n-th roots of unity up to translation. For even n, there will be n paths of zero area consisting of n/2 steps in one direction followed by n/2 steps in the opposite direction. %C A321415 Compared with A262181, this sequence counts all rotations distinctly and also for even n includes n/2 flat polygons as described above. %C A321415 For prime n, a(n) is always 2. For odd prime the two solutions are the one consisting of all n-th roots of unity and the other consisting of their negated values. %F A321415 a(p) = 2 for prime p. %e A321415 a(4) = 12 because there are 3 basic shapes illustrated below which with rotations of multiples of 45 degrees give 4 + 2 + 4 = 10 distinct convex paths. %e A321415 o---o o---o %e A321415 o===o===o | | \ \ %e A321415 o---o o---o %Y A321415 Main diagonal of A321414. %Y A321415 Cf. A262181, A292355. %K A321415 nonn,more %O A321415 1,2 %A A321415 _Andrew Howroyd_, Nov 08 2018