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 A208538 #15 Nov 01 2017 12:24:03 %S A208538 1,1,1,21,102,1505,19995,365260,7456596,174489813,4545454545, %T A208538 130773238871,4115123283810,140620807064413,5185603185296625, %U A208538 205262771447683860,8680820740569200760,390641235316599920745,18637772246193096746253,939749336469457562916217 %N A208538 Number of n-bead necklaces of n colors allowing reversal, with no adjacent beads having the same color. %H A208538 Andrew Howroyd, <a href="/A208538/b208538.txt">Table of n, a(n) for n = 1..80</a> %F A208538 a(2n+1) = A208533(2n+1)/2 for n > 0, a(2n) = (A208533(2n) + n*(2n-1)^n)/2. - _Andrew Howroyd_, Mar 12 2017 %e A208538 All solutions for n=4: %e A208538 ..1....1....1....1....2....1....1....1....2....1....1....3....2....2....1....1 %e A208538 ..2....4....3....2....3....2....3....3....4....3....2....4....3....4....2....2 %e A208538 ..4....2....2....3....2....4....1....4....2....2....1....3....2....3....1....1 %e A208538 ..2....4....4....2....3....3....3....3....4....3....3....4....4....4....2....4 %e A208538 .. %e A208538 ..1....1....2....1....1 %e A208538 ..3....4....3....2....4 %e A208538 ..1....3....4....3....1 %e A208538 ..4....4....3....4....4 %t A208538 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, n]; Array[a, 20] (* _Jean-François Alcover_, Nov 01 2017, after _Andrew Howroyd_ *) %Y A208538 Diagonal of A208544. %K A208538 nonn %O A208538 1,4 %A A208538 _R. H. Hardin_, Feb 27 2012 %E A208538 a(12)-a(20) from _Andrew Howroyd_, Mar 12 2017