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A000940 Number of n-gons with n vertices.
(Formerly M1260 N0482)
10
1, 2, 4, 12, 39, 202, 1219, 9468, 83435, 836017, 9223092, 111255228, 1453132944, 20433309147, 307690667072, 4940118795869, 84241805734539, 1520564059349452, 28963120073957838, 580578894859915650, 12217399235411398127, 269291841184184374868, 6204484017822892034404 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,2

COMMENTS

Number of inequivalent undirected Hamiltonian cycles in complete graph on n labeled nodes under action of dihedral group of order 2n acting on nodes.

After a(4) = 2 there are no primes through a(100). - Jonathan Vos Post, Feb 06 2011

REFERENCES

J. H. Kwak and J. Lee, Enumeration of graph coverings, surface branched coverings and related group theory, in Combinatorial and Computational Mathematics (Pohang, 2000), ed. S. Hong et al., World Scientific, Singapore 2001, pp. 97-161.

R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, Department of Mathematics, Univ. London, 1958.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

T. D. Noe, Table of n, a(n) for n = 3..100

S. W. Golomb and L. R. Welch, On the enumeration of polygons, Amer. Math. Monthly, 67 (1960), 349-353.

S. W. Golomb and L. R. Welch, On the enumeration of polygons, Amer. Math. Monthly, 67 (1960), 349-353. [Annotated scanned copy]

E. M. Palmer and R. W. Robinson, Enumeration under two representations of the wreath product, Acta Math., 131 (1973), 123-143.

R. C. Read, Letter to N. J. A. Sloane, Feb 04 1971 (gives initial terms of this sequence, except he has a(6)=7 instead of 12)

R. C. Read, Letter to N. J. A. Sloane, 1992

R. C. Read, Combinatorial problems in theory of music, Discrete Math. 167 (1997), 543-551.

N. J. A. Sloane, Illustration of initial terms [Annotated page from Golomb-Welch article]

Venta Terauds, J. Sumner, Circular genome rearrangement models: applying representation theory to evolutionary distance calculations, arXiv preprint arXiv:1712.00858 [q-bio.PE], 2017.

FORMULA

For formula see Maple lines.

EXAMPLE

Label the vertices of a regular n-gon 1,2,...,n.

For n=3,4,5 representatives for the polygons counted here are:

(1,2,3,1),

(1,2,3,4,1), (1,2,4,3,1),

(1,2,3,4,5,1), (1,2,3,5,4,1), (1,2,4,5,3,1), (1,3,5,2,4,1).

For n=6:

(1,2,3,4,5,6,1), (1,2,3,4,6,5,1), (1,2,3,5,6,4,1), (1,2,3,6,5,4,1),(1,2,4,3,6,5,1), (1,2,4,6,3,5,1), (1,2,4,6,5,3,1), (1,2,5,3,6,4,1),(1,2,5,4,6,3,1), (1,2,5,6,3,4,1), (1,2,6,4,5,3,1), (1,3,5,2,6,4,1).

MAPLE

with(numtheory);

# for n odd:

Sd:=proc(n) local t1, d; t1:=2^((n-1)/2)*n^2*((n-1)/2)!; for d from 1 to n do if n mod d = 0 then t1:=t1+phi(n/d)^2*d!*(n/d)^d; fi; od: t1/(4*n^2); end;

# for n even:

Se:=proc(n) local t1, d; t1:=2^(n/2)*n*(n+6)*(n/2)!/4; for d from 1 to n do if n mod d = 0 then t1:=t1+phi(n/d)^2*d!*(n/d)^d; fi; od: t1/(4*n^2); end;

A000940:=n-> if n mod 2 = 0 then Se(n) else Sd(n); fi;

MATHEMATICA

a[n_] := (t1 = If[OddQ[n], 2^((n - 1)/2)*n^2*((n - 1)/2)!, 2^(n/2)*n*(n + 6)*(n/2)!/4]; For[ d = 1 , d <= n, d++, If[Mod[n, d] == 0, t1 = t1 + EulerPhi[n/d]^2*d!*(n/d)^d]]; t1/(4*n^2)); Table[a[n], {n, 3, 25}] (* Jean-Fran├žois Alcover, Jun 19 2012, after Maple *)

PROG

(PARI) a(n)={if(n<3, 0, (2^(n\2-2)*(n\2)!*n*if(n%2, 4*n, n + 6) + sumdiv(n, d, eulerphi(n/d)^2*d!*(n/d)^d))/(4*n^2))} \\ Andrew Howroyd, Sep 09 2018

CROSSREFS

Cf. A000939. Bisections give A094156, A094157.

For permutation classes under various symmetries see A089066, A262480, A002619.

Sequence in context: A268069 A215071 A180487 * A008404 A170815 A170814

Adjacent sequences:  A000937 A000938 A000939 * A000941 A000942 A000943

KEYWORD

nonn,easy,nice

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Pab Ter (pabrlos(AT)yahoo.com), May 05 2004

STATUS

approved

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Last modified November 16 04:54 EST 2018. Contains 317257 sequences. (Running on oeis4.)