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 A215336 Cyclically smooth Lyndon words with 4 colors. 2
 4, 3, 6, 11, 26, 52, 124, 275, 648, 1511, 3618, 8635, 20920, 50758, 124114, 304425, 750330, 1854716, 4600692, 11441298, 28528484, 71290791, 178529666, 447914775, 1125756830, 2833896220, 7144466184, 18036398490, 45591671450, 115381759707, 292329164908, 741410952975, 1882219946418, 4782782372655, 12163730636096 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS We call a Lyndon word (x[1],x[2],...,x[n]) smooth if abs(x[k]-x[k-1]) <= 1 for 2<=k<=n, and cyclically smooth if abs(x[1]-x[n]) <= 1. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..200 Arnold Knopfmacher, Toufik Mansour, Augustine Munagi, Helmut Prodinger, Smooth words and Chebyshev polynomials, arXiv:0809.0551 [math.CO], 2008. FORMULA a(n) = sum_{ d divides n } moebius(n/d) * A208773(d). EXAMPLE The cyclically smooth necklaces (N) and Lyndon words (L) of length 4 with 4 colors (using symbols ".", "1", "2", and "3") are:     ....   1       .  N     ...1   4    ...1  N L     ..11   4    ..11  N L     .1.1   2      .1  N     .111   4    .111  N L     .121   4    .121  N L     1111   1       1  N     1112   4    1112  N L     1122   4    1122  N L     1212   2      12  N     1222   4    1222  N L     1232   4    1232  N L     2222   1       2  N     2223   4    2223  N L     2233   4    2233  N L     2323   2      23  N     2333   4    2333  N L     3333   1       3  N There are 18 necklaces (so A208773(4)=24) and a(4)=11 Lyndon words. MATHEMATICA terms = 40; sn[n_, k_] := 1/n Sum[EulerPhi[j] (1+2Cos[i Pi/(k+1)])^(n/j), {i, 1, k}, {j, Divisors[n]}]; vn = Table[Round[sn[n, 4]], {n, terms}]; vl = Table[Sum[MoebiusMu[n/d] vn[[d]], {d, Divisors[n]}], {n, terms}] (* Jean-François Alcover, Jul 22 2018, after Joerg Arndt *) PROG (PARI) default(realprecision, 99); /* using floats */ sn(n, k)=1/n*sum(i=1, k, sumdiv(n, j, eulerphi(j)*(1+2*cos(i*Pi/(k+1)))^(n/j))); vn=vector(66, n, round(sn(n, 4)) ); /* necklaces */ /* Lyndon words, via Moebius inversion: */ vl=vector(#vn, n, sumdiv(n, d, moebius(n/d)*vn[d])) CROSSREFS Cf. A208773 (cyclically smooth necklaces, 4 colors). Cf. A215329 (smooth necklaces, 4 colors), A215330 (smooth Lyndon words, 4 colors). Sequence in context: A196889 A005522 A276202 * A232328 A276229 A077955 Adjacent sequences:  A215333 A215334 A215335 * A215337 A215338 A215339 KEYWORD nonn AUTHOR Joerg Arndt, Aug 13 2012 STATUS approved

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Last modified November 15 14:03 EST 2018. Contains 317239 sequences. (Running on oeis4.)