|
|
A160873
|
|
Number of isomorphism classes of connected (D_4)-fold coverings of a connected graph with circuit rank n.
|
|
2
|
|
|
0, 3, 42, 420, 3720, 31248, 256032, 2072640, 16679040, 133824768, 1072169472, 8583644160, 68694312960, 549655154688, 4397643866112, 35182761492480, 281468534292480, 2251774043947008, 18014295430397952, 144114775759257600
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,2
|
|
COMMENTS
|
Previous name: "Regular coverings having dihedral voltage groups: see Kwak-Lee reference in A160870 for precise definition."
Also, number of isomorphism classes of connected (C_4 x C_2)-fold coverings of a connected graph with circuit rank n.
Also, number of lattices L in Z^n such that the quotient group Z^n / L is C_4 x C_2.
Also, number of subgroups of (C_4)^n isomorphic to C_4 x C_2.
(End)
|
|
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.
|
|
LINKS
|
|
|
FORMULA
|
a(n) = 2^(n-2) * (2^n - 1) * (2^(n-1) - 1) = 8^(n-1) - 6*4^(n-2) + 2^(n-2) [Kwak, Chun, and Lee]. - Álvar Ibeas, Oct 30 2015
a(n) = 2^(-3+n) * (2-3*2^n+4^n).
a(n) = 14*a(n-1)-56*a(n-2)+64*a(n-3) for n>3.
G.f.: -3*x^2/((2*x-1)*(4*x-1)*(8*x-1)). (End)
|
|
MATHEMATICA
|
Table[2^(-3+n)*(2-3*2^n+4^n), {n, 1, 30}] (* or *) LinearRecurrence[{14, -56, 64}, {0, 3, 42}, 30] (* G. C. Greubel, Apr 30 2018 *)
|
|
PROG
|
(PARI) a(n) = 2^(-3+n) * (2-3*2^n+4^n) \\ Colin Barker, Oct 30 2015
(PARI) concat(0, Vec(-3*x^2/((2*x-1)*(4*x-1)*(8*x-1)) + O(x^30))) \\ Colin Barker, Oct 30 2015
(Magma) [2^(-3+n)*(2-3*2^n+4^n): n in [1..30]]; // G. C. Greubel, Apr 30 2018
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,easy
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|