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 A208540 #23 Apr 21 2020 03:06:25 %S A208540 4,6,4,21,24,92,156,498,1096,3210,8052,22913,61320,173088,478316, %T A208540 1351983,3798240,10781954,30585828,87230157,249056136,713387076, %U A208540 2046590844,5884491500,16945772208,48883660146,141214768972 %N A208540 Number of n-bead necklaces of 4 colors allowing reversal, with no adjacent beads having the same color. %H A208540 Andrew Howroyd, <a href="/A208540/b208540.txt">Table of n, a(n) for n = 1..100</a> %H A208540 Marko Riedel et al., math.stackexchange, <a href="https://math.stackexchange.com/questions/2164399/">Proper colorings of necklaces</a> %H A208540 Marko Riedel et al., math.stackexchange, <a href="https://math.stackexchange.com/questions/3633494/">Proper colorings of bracelets</a> %F A208540 a(2*n+1) = A106366(2*n+1)/2 for n > 0, a(2*n) = (A106366(2*n) + 2*3^n)/2. - _Andrew Howroyd_, Mar 12 2017 %e A208540 All solutions for n=3 %e A208540 ..1....1....2....1 %e A208540 ..2....2....3....3 %e A208540 ..3....4....4....4 %t A208540 T[n_, k_] := If[n == 1, k, (DivisorSum[n, EulerPhi[n/#]*(k - 1)^# &]/n + If[OddQ[n], 1 - k, k*(k - 1)^(n/2)/2])/2]; a[n_] = T[n, 4]; Array[a, 27] (* _Jean-François Alcover_, Nov 01 2017, after _Andrew Howroyd_ *) %Y A208540 Column 4 of A208544. %K A208540 nonn %O A208540 1,1 %A A208540 _R. H. Hardin_, Feb 27 2012