

A228766


Number of undirected circular permutations i_1,...,i_{n1} of 1,...,n1 with i_1 + i_2, i_2 + i_3, ..., i_{n2} + i_{n1}, i_{n1} + i_1 pairwise distinct modulo n.


6




OFFSET

3,6


COMMENTS

Conjecture: a(n) > 0 for all n > 3. In general, if a_1,...,a_n are n > 2 distinct elements of a finite additive abelian group G with n odd or G not divisible by n, then there exists a circular permutation b_1,...,b_n of a_1,...,a_n such that b_1+b_2, b_2+b_3, ..., b_{n1}+b_n, b_n+b_1 are pairwise distinct.
Note that if g is a primitive root modulo a prime p > 3 then 1+g, g+g^2, ..., g^{p3}+g^{p2}, g^{p2}+1 are pairwise distinct modulo p. So a(p) > 0 for any prime p > 3.
If n > 2 is odd, then 0+1, 1+2, ..., (n2)+(n1), (n1)+0 are pairwise distinct modulo n, and hence the conjecture holds in the case {a_1,...,a_n} = G = Z/nZ.


LINKS

Table of n, a(n) for n=3..11.
Z.W. Sun, Some new problems in additive combinatorics, arXiv preprint arXiv:1309.1679, 2013


EXAMPLE

a(4) = 1 due to the circular permutation (1,2,3).
a(5) = 1 due to the circular permutation (1,2,4,3).
a(6) = 1 due to the circular permutation (1,3,5,2,4).
a(7) = 1 due to the circular permutation (1,3,2,6,4,5).
a(8) = 12 due to the circular permutations
(1,2,4,5,3,7,6), (1,2,6,7,3,4,5), (1,2,7,6,4,3,5), (1,4,2,5,6,3,7), (1,4,2,7,3,5,6), (1,4,3,7,2,6,5), (1,4,7,3,6,2,5), (1,5,2,3,6,4,7), (1,5,3,2,7,4,6), (1,5,4,7,3,2,6), (1,5,6,4,3,2,7), (1,6,5,4,2,3,7).
a(9) > 0 due to the permutation (1,2,3,4,6,5,8,7).
a(10) > 0 due to the permutation (1,2,4,5,6,8,9,3,7).
a(11) > 0 due to the permutation (1,2,3,4,6,7,5,10,9,8).


MATHEMATICA

(* A program to compute required circular permutations for n = 9. To get "undirected" circular permutations, we should identify a circular permutation with the one of the opposite direction; for example, (1, 7, 8, 5, 6, 4, 3, 2) is identitical to (1, 2, 3, 4, 6, 5, 8, 7) if we ignore direction. *)
V[i_]:=Part[Permutations[{2, 3, 4, 5, 6, 7, 8}], i]
m=0
Do[If[Length[Union[{Mod[1+Part[V[i], 1], 9]}, Table[Mod[Part[V[i], j]+If[j<7, Part[V[i], j+1], 1], 9], {j, 1, 7}]]]<8, Goto[aa]];
m=m+1; Print[m, ":", " ", 1, " ", Part[V[i], 1], " ", Part[V[i], 2], " ", Part[V[i], 3], " ", Part[V[i], 4], " ", Part[V[i], 5], " ", Part[V[i], 6], " ", Part[V[i], 7]]; Label[aa]; Continue, {i, 1, 7!}]


CROSSREFS

Cf. A228762, A228772, A185645, A228728.
Sequence in context: A219542 A137480 A316267 * A157157 A218043 A048344
Adjacent sequences: A228763 A228764 A228765 * A228767 A228768 A228769


KEYWORD

nonn,more


AUTHOR

ZhiWei Sun, Sep 03 2013


STATUS

approved



