Efficient Methods in Counting Generalized Necklaces

Abstract

It is shown in [7] by Venkaiah in 2015 that a category of the number of generalized can be computed using the expression \begin{equation*} e(n, q) = \frac{1}{(q-1) ord(\lambda) n} \sum^{ord(\lambda)n}_{\substack{t \in \mathbb{F}_q \setminus \{0\}, i=1 \\ t^{\frac{n}{\gcd(n, i)}} \lambda^{\frac{i}{\gcd(n,i)}} = 1}}(q^{\gcd(n,i)} - 1) + 1 \end{equation*} where $q$ (number of colors) is the size of the prime field $\mathbb{F}_q$, $\lambda$ is the constant of the consta-cyclic shift, $n$ is the length of the necklace. However, direct evaluation of this expression requires, apart from the $\gcd$ computations, $2*(q-1)*Ord(\lambda)*n$ exponentiations and $(q-1)*Ord(\lambda)*n$ multiplications, at most $(q-1)*Ord(\lambda)*n$ exponentiations and at most $2*(q-1)*Ord(\lambda)*n$ additions and hence computationally intensive. This note discusses various other ways of evaluating the expression and tries to throw some light on amortizing the amount of computation.