# Cyclotomic fast Fourier transform

The cyclotomic fast Fourier transform is a type of fast Fourier transform algorithm over finite fields.[1] This algorithm first decomposes a DFT into several circular convolutions, and then derives the DFT results from the circular convolution results. When applied to a DFT over GF(2m), this algorithm has a very low multiplicative complexity. In practice, since there usually exist efficient algorithms for circular convolutions with specific lengths, this algorithm is very efficient.[2]

## Background

The discrete Fourier transform over finite fields finds widespread application in the decoding of error-correcting codes such as BCH codes and Reed–Solomon codes. Generalized from the complex field, a discrete Fourier transform of a sequence $\{f_i\}_{0}^{N-1}$ over a finite field GF(pm) is defined as

$F_j=\sum_{i=0}^{N-1}f_i\alpha^{ij}, 0\le j\le N-1,$

where $\alpha$ is the N-th primitive root of 1 in GF(pm). If we define the polynomial representation of $\{f_i\}_{0}^{N-1}$ as

$f(x) = f_0+f_1x+f_2x^2+\cdots+f_{N-1}x^{N-1}=\sum_{0}^{N-1}f_ix^i,$

it is easy to see that $F_j$ is simply $f(\alpha^j)$. That is, the discrete Fourier transform of a sequence converts it to a polynomial evaluation problem.

Written in matrix format,

$\mathbf{F}=\left[\begin{matrix}F_0\\F_1\\ \vdots \\ F_{N-1}\end{matrix}\right]= \left[\begin{matrix} \alpha^0&\alpha^0 &\cdots & \alpha^0\\ \alpha^0 & \alpha^1 &\cdots &\alpha^{N-1}\\ \vdots &\vdots & \ddots & \vdots \\ \alpha^{0} & \alpha^{N-1} &\cdots & \alpha^{(N-1)(N-1)} \end{matrix}\right] \left[\begin{matrix}f_0\\f_1\\\vdots\\f_{N-1}\end{matrix}\right]=\mathcal{F}\mathbf{f}.$

Direct evaluation of DFT has an $O(N^2)$ complexity. Fast Fourier transforms are just efficient algorithms evaluating the above matrix-vector product.

## Algorithm

First, we define a linearized polynomial over GF(pm) as

$L(x) = \sum_{i} l_i x^{p^i}, l_i \in \mathrm{GF}(p^m).$

$L(x)$ is called linearized because $L(x_1+x_2) = L(x_1) + L(x_2)$, which comes from the fact that for elements $x_1,x_2 \in \mathrm{GF}(p^m),$$(x_1+x_2)^p=x_1^p+x_2^p.$

Notice that $p$ is invertible modulo $N$ because $N$ must divide the order $p^m-1$ of the multiplicative group of the field $\mathrm{GF}(p^m)$. So, the elements $\{0, 1, 2, \ldots, N-1\}$ can be partitioned into $l+1$ cyclotomic cosets modulo $N$:

$\{0\},$
$\{k_1, pk_1, p^2k_1, \ldots, p^{m_1-1}k_1\},$
$\ldots,$
$\{k_l, pk_l, p^2k_l, \ldots, p^{m_l-1}k_l\},$

where $k_i=p^{m_i}k_i \pmod{N}$. Therefore, the input to the Fourier transform can be rewritten as

$f(x)=\sum_{i=0}^l L_i(x^{k_i}),\quad L_i(y) = \sum_{t=0}^{m_i-1}y^{p^t}f_{p^tk_i\bmod{N}}.$

In this way, the polynomial representation is decomposed into a sum of linear polynomials, and hence $F_j$ is given by

$F_j=f(\alpha^j)=\sum_{i=0}^lL_i(\alpha^{jk_i})$.

Expanding $\alpha^{jk_i} \in \mathrm{GF}(p^{m_i})$ with the proper basis $\{\beta_{i,0}, \beta_{i,1}, \ldots, \beta_{i,m_i-1}\}$, we have $\alpha^{jk_i} = \sum_{s=0}^{m_i-1}a_{ijs}\beta_{i,s}$ where $a_{ijs} \in \mathrm{GF}(p)$, and by the property of the linearized polynomial $L_i(x)$, we have

$F_j=\sum_{i=0}^l\sum_{s=0}^{m_i-1}a_{ijs}\left(\sum_{t=0}^{m_i-1}\beta_{i,s}^{p^t}f_{p^{t}k_i\bmod{N}}\right)$

This equation can be rewritten in matrix form as $\mathbf{F}=\mathbf{AL\Pi f}$, where $\mathbf{A}$ is an $N\times N$ matrix over GF(p) that contains the elements $a_{ijs}$, $\mathbf{L}$ is a block diagonal matrix, and $\mathbf{\Pi}$ is a permutation matrix regrouping the elements in $\mathbf{f}$ according to the cyclotomic coset index.

Note that if the normal basis $\{\gamma_i^{p^0}, \gamma_i^{p^1}, \cdots, \gamma_i^{p^{m_i-1}}\}$ is used to expand the field elements of $\mathrm{GF}(p^{m_i})$, the i-th block of $\mathbf{L}$ is given by:

$\mathbf{L}_i= \begin{bmatrix} \gamma_i^{p^0} &\gamma_i^{p^1} &\cdots &\gamma_i^{p^{m_i-1}}\\ \gamma_i^{p^1} &\gamma_i^{p^2} &\cdots &\gamma_i^{p^{0}}\\ \vdots & \vdots & \ddots & \vdots\\ \gamma_i^{p^{m_i-1}} &\gamma_i^{p^0} &\cdots &\gamma_i^{p^{m_i-2}}\\ \end{bmatrix}$

which is a circulant matrix. It is well known that a circulant matrix-vector product can be efficiently computed by convolutions. Hence we successfully reduce the discrete Fourier transform into short convolutions.

## Complexity

When applied to a characteristic-2 field GF(2m), the matrix $\mathbf{A}$ is just a binary matrix. Only addition is used when calculating the matrix-vector product of $\mathrm{A}$ and $\mathrm{L\Pi f}$. It has been shown that the multiplicative complexity of the cyclotomic algorithm is given by $O(n(\log_2n)^{\log_2\frac{3}{2}})$, and the additive complexity is given by $O(n^2/(\log_2 n)^{\log_2\frac{8}{3}})$.[2]

## References

1. ^ S.V. Fedorenko and P.V. Trifonov, Fedorenko, S. V.; Trifonov, P. V.. (2003). "On Computing the Fast Fourier Transform over Finite Fields". Proceedings of International Workshop on Algebraic and Combinatorial Coding Theory: 108–111.
2. ^ a b Wu, Xuebin; Wang, Ying; Yan, Zhiyuan (2012). "On Algorithms and Complexities of Cyclotomic Fast Fourier Transforms Over Arbitrary Finite Fields". IEEE Transactions on Signal Processing 60 (3): 1149–1158. doi:10.1109/tsp.2011.2178844.