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 A058880 Number of orientations of an n-cycle. 1
 12, 36, 104, 352, 1172, 4119, 14572, 52492, 190652, 699266, 2581112, 9587602, 35791472, 134219859, 505290272, 1908881998, 7233629132, 27487817244, 104715393912, 399822505942, 1529755308212, 5864062368274, 22517998136936 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,1 REFERENCES F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 129, (5.3.3). LINKS Amiram Eldar, Table of n, a(n) for n = 3..1666 FORMULA a(n) = (1/(2n)) * Sum_{d|n} phi(d) * 2^(2n/d) + (2^((n-4)/2), if n is even). - Amiram Eldar, Aug 28 2019 MAPLE A058880 := proc(n) local d, t1, t2; if n mod 2 = 0 then t1 := 2^((n-4)/2) else t1 := 0; fi; t2 := divisors(n); for d in t2 do t1 := t1+phi(d)*2^(2*n/d)/(2*n); od; t1; end; MATHEMATICA a[n_] := DivisorSum[n, EulerPhi[#]*2^(2n/#) &]/(2n) + If[OddQ[n], 0, 2^((n - 4)/2)]; Array[a, 23, 3] (* Amiram Eldar, Aug 28 2019 *) PROG (PARI) a(n) = sumdiv(n, d, eulerphi(d)*2^(2*n/d))/(2*n) + if (!(n%2), 2^((n-4)/2)); \\ Michel Marcus, Aug 29 2019 CROSSREFS Sequence in context: A172218 A172212 A060621 * A282097 A055551 A073403 Adjacent sequences:  A058877 A058878 A058879 * A058881 A058882 A058883 KEYWORD nonn AUTHOR N. J. A. Sloane, Jan 07 2001 STATUS approved

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Last modified August 3 20:08 EDT 2020. Contains 336201 sequences. (Running on oeis4.)