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 A056665 #44 Sep 11 2022 09:29:27
%S A056665 1,3,11,70,629,7826,117655,2097684,43046889,1000010044,25937424611,
%T A056665 743008623292,23298085122493,793714780783770,29192926025492783,
%U A056665 1152921504875290696,48661191875666868497,2185911559749720272442,104127350297911241532859
%N A056665 Number of equivalence classes of n-valued Post functions of 1 variable under action of complementing group C(1,n).
%C A056665 Diagonal of arrays defined in A054630 and A054631.
%C A056665 Given n colors, a(n) = number of necklaces with n beads and 1 up to n colors effectively assigned to them (super_labeled: which also generates n different monochrome necklaces). - _Wouter Meeussen_, Aug 09 2002
%C A056665 Number of endofunctions on a set with n objects up to cyclic permutation (rotation). E.g., for n = 3, the 11 endofunctions are 1,1,1; 2,2,2; 3,3,3; 1,1,2; 1,2,2; 1,1,3; 1,3,3; 2,2,3; 2,3,3; 1,2,3; and 1,3,2. - _Franklin T. Adams-Watters_, Jan 17 2007
%C A056665 Also number of pre-necklaces in Sigma(n,n) (see Ruskey and others). - _Peter Luschny_, Aug 12 2012
%C A056665 From _Olivier Gérard_, Aug 01 2016: (Start)
%C A056665 Decomposition of the endofunctions by class size.
%C A056665 .
%C A056665 n | 1 2 3 4 5 6 7
%C A056665 --+----------------------------------
%C A056665 1 | 1
%C A056665 2 | 2 1
%C A056665 3 | 3 0 8
%C A056665 4 | 4 6 0 60
%C A056665 5 | 5 0 0 0 624
%C A056665 6 | 6 15 70 0 0 7735
%C A056665 7 | 7 0 0 0 0 0 117648
%C A056665 .
%C A056665 The right diagonal gives the number of Lyndon Words or aperiodic necklaces, A075147. By multiplying each column by the corresponding size and summing, one gets A000312.
%C A056665 (End)
%D A056665 D. E. Knuth. Generating All Tuples and Permutations. The Art of Computer Programming, Vol. 4, Fascicle 2, 7.2.1.1. Addison-Wesley, 2005.
%H A056665 Alois P. Heinz, <a href="/A056665/b056665.txt">Table of n, a(n) for n = 1..200</a>
%H A056665 M. A. Harrison and R. G. High, <a href="http://dx.doi.org/10.1016/S0021-9800(68)80008-0">On the cycle index of a product of permutation groups</a>, J. Combin. Theory, 4 (1968), 277-299.
%H A056665 F. Ruskey, C. Savage, and T. M. Y. Wang, <a href="http://dx.doi.org/10.1016/0196-6774(92)90047-G">Generating necklaces</a>, Journal of Algorithms, 13(3), 414 - 430, 1992.
%H A056665 <a href="/index/Gre#groups">Index entries for sequences related to groups</a>
%F A056665 a(n) = Sum_{d|n} phi(d)*n^(n/d)/n.
%F A056665 a(n) ~ n^(n-1). - _Vaclav Kotesovec_, Sep 11 2014
%F A056665 a(n) = (1/n) * Sum_{k=1..n} n^gcd(k,n). - _Joerg Arndt_, Mar 19 2017
%F A056665 a(n) = [x^n] -Sum_{k>=1} phi(k)*log(1 - n*x^k)/k. - _Ilya Gutkovskiy_, Mar 21 2018
%F A056665 From _Richard L. Ollerton_, May 07 2021: (Start)
%F A056665 a(n) = (1/n)*Sum_{k=1..n} n^(n/gcd(n,k))*phi(gcd(n,k))/phi(n/gcd(n,k)).
%F A056665 a(n) = (1/n)*A228640(n). (End)
%e A056665 The 11 necklaces for n=3 are (grouped by partition of 3): (RRR,GGG,BBB),(RRG,RGG, RRB,RBB, GGB,GBB), (RGB,RBG).
%p A056665 with(numtheory):
%p A056665 a:= n-> add(phi(d)*n^(n/d), d=divisors(n))/n:
%p A056665 seq(a(n), n=1..25); # _Alois P. Heinz_, Jun 18 2013
%t A056665 Table[Fold[ #1+EulerPhi[ #2] n^(n/#2)&, 0, Divisors[n]]/n, {n, 7}]
%o A056665 (Sage)
%o A056665 # This algorithm counts all n-ary n-tuples (a_1,..,a_n) such that the string a_1...a_n is preprime. It is algorithm F in Knuth 7.2.1.1.
%o A056665 def A056665_list(n):
%o A056665 C = []
%o A056665 for m in (1..n):
%o A056665 a = [0]*(n+1); a[0]=-1;
%o A056665 j = 1; count = 0
%o A056665 while(true):
%o A056665 if m%j == 0 : count += 1;
%o A056665 j = n
%o A056665 while a[j] >= m-1 : j -= 1
%o A056665 if j == 0 : break
%o A056665 a[j] += 1
%o A056665 for k in (j+1..n): a[k] = a[k-j]
%o A056665 C.append(count)
%o A056665 return C
%o A056665 (Sage)
%o A056665 def A056665(n): return sum(euler_phi(d)*n^(n//d)//n for d in divisors(n))
%o A056665 [A056665(n) for n in (1..18)] # _Peter Luschny_, Aug 12 2012
%o A056665 (PARI) a(n) = sum(k=1,n,n^gcd(k,n)) / n; \\ _Joerg Arndt_, Mar 19 2017
%Y A056665 Cf. A001323, A001324, A001372, A000312.
%Y A056665 Diagonal of arrays defined in A054630, A054631 and A075195.
%Y A056665 Cf. A075147 Aperiodic necklaces, a subset of this sequence.
%Y A056665 Cf. A000169 Classes under translation mod n
%Y A056665 Cf. A168658 Classes under complement to n+1
%Y A056665 Cf. A130293 Classes under translation and rotation
%Y A056665 Cf. A081721 Classes under rotation and reversal
%Y A056665 Cf. A275549 Classes under reversal
%Y A056665 Cf. A275550 Classes under reversal and complement
%Y A056665 Cf. A275551 Classes under translation and reversal
%Y A056665 Cf. A275552 Classes under translation and complement
%Y A056665 Cf. A275553 Classes under translation, complement and reversal
%Y A056665 Cf. A275554 Classes under translation, rotation and complement
%Y A056665 Cf. A275555 Classes under translation, rotation and reversal
%Y A056665 Cf. A275556 Classes under translation, rotation, complement and reversal
%Y A056665 Cf. A275557 Classes under rotation and complement
%Y A056665 Cf. A275558 Classes under rotation, complement and reversal
%Y A056665 Cf. A228640.
%K A056665 easy,nonn
%O A056665 1,2
%A A056665 _Vladeta Jovovic_, Aug 09 2000