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 A208590 #13 Nov 01 2017 12:24:49
%S A208590 1,3,13,125,1761,34153,833253,24627705,855798877,34223775091,
%T A208590 1549090700059,78325577416281,4376902102440375,267933660979057571,
%U A208590 17834397187887850661,1282662234529669344753,99133711906097886767313,8194555387462964814207631
%N A208590 Number of n-bead necklaces labeled with numbers -n..n not allowing reversal, with sum zero.
%H A208590 Andrew Howroyd, <a href="/A208590/b208590.txt">Table of n, a(n) for n = 1..80</a>
%e A208590 All solutions for n=3:
%e A208590 .-2...-1...-2...-3...-3...-2....0...-1...-1...-2...-3...-3...-2
%e A208590 ..0....1....3....2....1....1....0....0...-1....2....0....3...-1
%e A208590 ..2....0...-1....1....2....1....0....1....2....0....3....0....3
%t A208590 comps[r_, m_, k_] := Sum[(-1)^i*Binomial[r - 1 - i*m, k - 1]*Binomial[k, i], {i, 0, Floor[(r - k)/m]}]; a[n_, k_] := DivisorSum[n, EulerPhi[n/#] comps[#*(k + 1), 2 k + 1, #] &]/n; a[n_] := a[n, n]; Array[a, 18] (* _Jean-François Alcover_, Nov 01 2017, after _Andrew Howroyd_ *)
%Y A208590 Diagonal of A208597.
%K A208590 nonn
%O A208590 1,2
%A A208590 _R. H. Hardin_, Feb 29 2012
%E A208590 a(12)-a(18) from _Andrew Howroyd_, Mar 02 2017