OFFSET
1,2
COMMENTS
See A212355 for the formula for the cycle index Z(D_n) of the dihedral group, the Harary and Palmer reference, and a link for these polynomials for n=1..15.
It seems that this is also the number of different sets of distances of n points placed on 2n equidistant points on a circle. - M. F. Hasler, Jan 28 2013
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..1001
FORMULA
EXAMPLE
a(6) = 5, because tau(6) = 4. The row no. 6 of A212355 is [2,0,0,2,0,0,4,0,3,0,1] with 5 non-vanishing entries.
Illustration of a(7)=3 = number of different sets of distances of 7 points among {z=e^(i k pi/7), k=0..13}: Inequivalent configurations are, e.g.: [k]=[0,2,4,6,8,10,12] with distances {0.86777, 1.5637, 1.9499}, [k]=[0,1,2,3,4,5,6] with distances {0.44504, 0.86777, 1.2470, 1.5637, 1.8019, 1.9499}, and [k]=[0,1,2,3,4,5,7] with distances {0.44504, 0.86777, 1.2470, 1.5637, 1.8019, 1.9499, 2.0000}. - M. F. Hasler, Jan 28 2013
PROG
(PARI) A212356(n) = if(n<=2, n, 1+numdiv(n)); \\ Antti Karttunen, Sep 22 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Jun 02 2012
STATUS
approved