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A320749
Number of chiral pairs of color patterns (set partitions) in a cycle of length n.
4
0, 0, 0, 0, 0, 6, 34, 190, 1011, 5352, 29740, 172466, 1055232, 6793791, 46034940, 327303819, 2436650368, 18944771253, 153488081102, 1293086505784, 11306373089104, 102425178180769, 959825673145688, 9290807818971900, 92771800581171418, 954447025978145744, 10105871186441842623, 110009631951698573068, 1229996584263621368224, 14112483571723367245825, 166021918475962174194914, 2001010469483653602192695
OFFSET
1,6
COMMENTS
Two color patterns are equivalent if the colors are permuted.
Adnk[d,n,k] in Mathematica program is coefficient of x^k in A(d,n)(x) in Gilbert and Riordan reference.
LINKS
E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois J. Math., 5 (1961), 657-665.
FORMULA
a(n) = Sum_{j=1..n} -Ach(n,j)/2 + (1/2n)*Sum_{d|n} phi(d)*A(d,n/d,j), where Ach(n,k) = [n>=0 & n<2 & n==k] + [n>1]*(k*Ach(n-2,k) + Ach(n-2,k-1) + Ach(n-2,k-2)) and A(d,n,k) = [n==0 & k==0] + [n>0 & k>0]*(k*A(d,n-1,k) + Sum_{j|d} A(d,n-1,k-j)).
a(n) = (A084423(n) - A080107(n)) / 2 = A084423(n) - A084708(n) = A084708(n) - A080107(n).
EXAMPLE
For a(6)=6, the chiral pairs are AAABBC-AAABCC, AABABC-AABCAC, AABACB-AABCAB, AABACC-AABBAC, AABACD-AABCAD, and AABCBD-AABCDC.
MATHEMATICA
Adnk[d_, n_, k_] := Adnk[d, n, k] = If[n>0 && k>0, Adnk[d, n-1, k]k + DivisorSum[d, Adnk[d, n-1, k-#]&], Boole[n==0 && k==0]]
Ach[n_, k_] := Ach[n, k] = If[n<2, Boole[n==k && n>=0], k Ach[n-2, k] + Ach[n-2, k-1] + Ach[n-2, k-2]]
Table[Sum[(DivisorSum[n, EulerPhi[#] Adnk[#, n/#, j]&]/n - Ach[n, j])/2, {j, n}], {n, 40}]
CROSSREFS
Row sums of A320647.
Columns of A320742 converge to this as k increases.
Cf. A084423 (oriented), A084708 (unoriented), A080107 (achiral).
Sequence in context: A125343 A163350 A320746 * A052264 A049608 A244937
KEYWORD
nonn,easy
AUTHOR
Robert A. Russell, Oct 22 2018
STATUS
approved