Necklace (combinatorics)

There are

${\displaystyle N_{k}(n)={\frac {1}{n}}\sum _{d\mid n}\varphi (d)k^{n/d}={\frac {1}{n}}\sum _{i=1}^{n}k^{\,{\rm {gcd}}(i,n)}}$ 

different k-ary necklaces of length n, where ${\displaystyle \varphi }$  is Euler's totient function.^[1] When the beads are restricted to particular color multiset ${\displaystyle {\mathcal {B}}=\{1^{n_{1}},\ldots ,k^{n_{k}}\}}$ , where ${\displaystyle n_{i}}$  is the number of beads of color ${\displaystyle i\in \{1,\ldots ,k\}}$ , there are

${\displaystyle N({\mathcal {B}})={\frac {1}{|{\mathcal {B}}|}}\sum _{d|{\rm {gcd}}(n_{1},\ldots ,n_{k})}{|{\mathcal {B}}|/d \choose n_{1}/d,\ldots ,n_{k}/d}\phi (d)}$ 

different necklaces made of all the beads of ${\displaystyle {\mathcal {B}}}$ . ^[2] Here ${\displaystyle |{\mathcal {B}}|:=\sum \limits _{i=1}^{k}n_{i}}$  and ${\displaystyle {m \choose m_{1},\ldots ,m_{k}}:={\frac {m!}{m_{1}!\cdots m_{k}!}}}$  is the multinomial coefficient. These two formulas follow directly from Pólya's enumeration theorem applied to the action of the cyclic group ${\displaystyle C_{n}}$  acting on the set of all functions ${\displaystyle f:\{1,\ldots ,n\}\to \{1,\ldots ,k\}}$ . If all k colors must be used, the count is

${\displaystyle L_{k}(n)={\frac {k!}{n}}\sum _{d\mid n}\varphi (d)S\left({\frac {n}{d}},k\right)\;,}$ 

where ${\displaystyle S(n,k)}$  are the Stirling number of the second kind.

${\displaystyle N_{k}(n)}$  (sequence A054631 in the OEIS) and ${\displaystyle L_{k}(n)}$  (sequence A087854 in the OEIS) are related via the Binomial coefficients:

${\displaystyle N_{k}(n)=\sum _{j=1}^{k}{\binom {k}{j}}L_{j}(n)}$ 

and

${\displaystyle L_{k}(n)=\sum _{j=1}^{k}(-1)^{k-j}{\binom {k}{j}}N_{j}(n)}$ 

The number of different k-ary bracelets of length n (sequence A081720 in the OEIS) is

${\displaystyle B_{k}(n)={\begin{cases}{\tfrac {1}{2}}N_{k}(n)+{\tfrac {1}{4}}(k+1)k^{n/2}&{\text{if }}n{\text{ is even}}\\[10px]{\tfrac {1}{2}}N_{k}(n)+{\tfrac {1}{2}}k^{(n+1)/2}&{\text{if }}n{\text{ is odd}}\end{cases}}\quad ,}$ 

where N_k(n) is the number of k-ary necklaces of length n. This follows from Pólya's method applied to the action of the dihedral group ${\displaystyle D_{n}}$ .

 

For a given set of n beads, all distinct, the number of distinct necklaces made from these beads, counting rotated necklaces as the same, is ⁠n!/n⁠ = (n − 1)!. This is because the beads can be linearly ordered in n! ways, and the n circular shifts of such an ordering all give the same necklace. Similarly, the number of distinct bracelets, counting rotated and reflected bracelets as the same, is ⁠n!/2n⁠, for n ≥ 3.

If the beads are not all distinct, having repeated colors, then there are fewer necklaces (and bracelets). The above necklace-counting polynomials give the number necklaces made from all possible multisets of beads. Polya's pattern inventory polynomial refines the counting polynomial, using variable for each bead color, so that the coefficient of each monomial counts the number of necklaces on a given multiset of beads.

An aperiodic necklace of length n is a rotation equivalence class having size n, i.e., no two distinct rotations of a necklace from such class are equal.

According to Moreau's necklace-counting function, there are

${\displaystyle M_{k}(n)={\frac {1}{n}}\sum _{d\mid n}\mu (d)k^{n/d}}$ 

different k-ary aperiodic necklaces of length n, where μ is the Möbius function. The two necklace-counting functions are related by: ${\textstyle N_{k}(n)=\sum _{d|n}M_{k}(d),}$  where the sum is over all divisors of n, which is equivalent by Möbius inversion to ${\textstyle M_{k}(n)=\sum _{d|n}N_{k}(d)\,\mu {\bigl (}{\tfrac {n}{d}}{\bigr )}.}$ 

Each aperiodic necklace contains a single Lyndon word so that Lyndon words form representatives of aperiodic necklaces.

• Lyndon word
• Inversion (discrete mathematics)
• Necklace problem
• Necklace splitting problem
• Permutation
• Proofs of Fermat's little theorem#Proof by counting necklaces
• Forte number, a representation of binary bracelets of length 12 used in atonal music.

• Polya, Georg; Read, R.C.; Aeppli, Dorothee (1987). Combinatorial enumeration of groups, graphs, and chemical compounds. Springer-Verlag. ISBN 9780387964133.
• Ruskey, Frank; Savage, Carla; Wang, Terry Min Yih (1992). "Generating necklaces". Journal of Algorithms. 13 (3): 414–430. doi:10.1016/0196-6774(92)90047-G.