# Discrete Fourier transform (general)

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Discrete Fourier transform (general)

This article is about the discrete Fourier transform (DFT) over any field (including finite fields), commonly called a number-theoretic transform (NTT) in the case of finite fields. For specific information on the discrete Fourier transform over the complex numbers, see discrete Fourier transform.

## Definition

Let F be any field, let $n\geq 1$ be an integer, and let α be a primitive nth root of unity. We assume α lies in F.

The discrete Fourier transform maps an n-tuple $(v_0,\ldots,v_{n-1})$ of elements of F to another n-tuple $(f_0,\ldots,f_{n-1})$ of elements of F according to the following formula: $f_k = \sum_{j=0}^{n-1} v_j\alpha^{jk}.\qquad (1)$

By convention, the tuple $(v_0,\ldots,v_{n-1})$ is said to be in the time domain and the index j is called time. The tuple $(f_0,\ldots,f_{n-1})$ is said to be in the frequency domain and the index k is called frequency. The tuple $(f_0,\ldots,f_{n-1})$ is also called the spectrum of $(v_0,\ldots,v_{n-1})$. This terminology derives from the applications of Fourier transforms in signal processing.

## Inverse

The inverse of the discrete Fourier transform is given as: $v_j = \frac{1}{n}\sum_{k=0}^{n-1} f_k\alpha^{-jk}.\qquad (2)$

Proof: first, note that whenever β is an nth root of unity, then βn − 1 = 0. If moreover $\beta\neq 1$, then $\beta-1\neq 0$, and therefore $\frac{\beta^n-1}{\beta-1} = 1+\beta+\beta^2+\cdots+\beta^{n-1} = 0.\qquad (3)$

Substituting (1) into the right-hand-side of (2), we get \begin{align} & \frac{1}{n}\sum_{k=0}^{n-1} f_k\alpha^{-jk} \\ & {} = \frac{1}{n}\sum_{k=0}^{n-1}\sum_{j'=0}^{n-1} v_{j'}\alpha^{j'k}\alpha^{-jk} \\ & {} = \frac{1}{n}\sum_{j'=0}^{n-1} v_{j'} \sum_{k=0}^{n-1}\alpha^{(j'-j)k}. \end{align}

This is exactly equal to vj, because $\sum_{k=0}^{n-1}\alpha^{(j'-j)k}=0$ when $j'\neq j$ (by (3) with β = αj' − j), and $\sum_{k=0}^{n-1}\alpha^{(j'-j)k}=n$ when j' = j.

## Matrix formulation

Since the discrete Fourier transform is a linear operator, it can be described by matrix multiplication. In matrix notation, the discrete Fourier transform is expressed as follows: $\begin{bmatrix}f_0\\f_1\\\vdots\\f_{n-1}\end{bmatrix} = \begin{bmatrix} 1&1&1&\cdots &1 \\ 1&\alpha&\alpha^2&\cdots&\alpha^{n-1} \\ 1&\alpha^2&\alpha^4&\cdots&\alpha^{2(n-1)}\\ \vdots&\vdots&\vdots&&\vdots\\ 1&\alpha^{n-1}&\alpha^{2(n-1)}&\cdots&\alpha^{(n-1)(n-1)}\\ \end{bmatrix} \begin{bmatrix}v_0\\v_1\\\vdots\\v_{n-1}\end{bmatrix}.$

The matrix for this transformation is called the DFT matrix.

Similarly, the matrix notation for the inverse Fourier transform is $\begin{bmatrix}v_0\\v_1\\\vdots\\v_{n-1}\end{bmatrix} = \frac{1}{n}\begin{bmatrix} 1&1&1&\cdots &1 \\ 1&\alpha^{-1}&\alpha^{-2}&\cdots&\alpha^{-(n-1)} \\ 1&\alpha^{-2}&\alpha^{-4}&\cdots&\alpha^{-2(n-1)}\\ \vdots&\vdots&\vdots&&\vdots\\ 1&\alpha^{-(n-1)}&\alpha^{-2(n-1)}&\cdots&\alpha^{-(n-1)(n-1)}\\ \end{bmatrix} \begin{bmatrix}f_0\\f_1\\\vdots\\f_{n-1}\end{bmatrix}.$

## Polynomial formulation

Sometimes it is convenient to identify an n-tuple $(v_0,\ldots,v_{n-1})$ with a formal polynomial $p_v(x) = v_0 + v_1x + v_2x^2 + \cdots + v_{n-1}x^{n-1}. \,$

By writing out the summation in the definition of the discrete Fourier transform (1), we obtain: $f_k = v_0 + v_1\alpha^{k} + v_2\alpha^{2k} + \cdots + v_{n-1}\alpha^{(n-1)k}. \,$

This means that fk is just the value of the polynomial pv(x) for x = αk, i.e., $f_k = p_v(\alpha^k).\,$

The Fourier transform can therefore be seen to relate the coefficients and the values of a polynomial: the coefficients are in the time-domain, and the values are in the frequency domain. Here, of course, it is important that the polynomial is evaluated at the nth roots of unity, which are exactly the powers of α.

Similarly, the definition of the inverse Fourier transform (2) can be written: $v_j = \frac{1}{n}(f_0 + f_1\alpha^{-j} + f_2\alpha^{-2j} + \cdots + f_{n-1}\alpha^{-(n-1)j}).\qquad (5)$

With $p_f(x) = f_0 + f_1x + f_2x^2 + \cdots + f_{n-1}x^{n-1},$

this means that $v_j = \frac{1}{n}p_f(\alpha^{-j}).$

We can summarize this as follows: if the values of p(x) are the coefficients of q(x), then the values of q(x) are the coefficients of p(x), up to a scalar factor and reordering.

## Special cases

### Complex numbers

If $F={\mathbb C}$ is the field of complex numbers, then the nth roots of unity can be visualized as points on the unit circle of the complex plane. In this case, one usually takes $\alpha=e^{\frac{-2\pi i}{n}},$

which yields the usual formula for the complex discrete Fourier transform: $f_k = \sum_{j=0}^{n-1} v_j e^{\frac{-2\pi i}{n}jk}.$

Over the complex numbers, it is often customary to normalize the formulas for the DFT and inverse DFT by using the scalar factor $\frac{1}{\sqrt{n}}$ in both formulas, rather than 1 in the formula for the DFT and $\frac{1}{n}$ in the formula for the inverse DFT. With this normalization, the DFT matrix is then unitary. Note that $\sqrt{n}$ does not make sense in an arbitrary field.

### Finite fields

If F = GF(q) is a finite field, where q is a prime power, then the existence of a primitive nth root automatically implies that n divides q − 1 (because the multiplicative order of each element must divide the size of the multiplicative group of F, which is q − 1). This in particular ensures that $n=\underbrace{1+1+\cdots+1}_{n\ \rm times}$ is invertible, so that the notation $\frac{1}{n}$ in (2) makes sense.

An application of the discrete Fourier transform over GF(q) is the reduction of Reed-Solomon codes to BCH codes in coding theory.

### Number-theoretic transform

The so-called number-theoretic transform (NTT) is obtained by specializing the discrete Fourier transform to $F={\mathbb Z}_p$, the integers modulo a prime p. In this case, primitive nth roots of unity exists whenever n divides p − 1, so we have p = ξn + 1 for a positive integer ξ. Specifically, let ω be a primitive (p − 1)st root of unity, then an nth root of unity α can be found by letting α = ωξ.

The number-theoretic transform can further be generalized to operate on elements of the ring $\mathbb{Z}_m$, even when the modulus m is not prime. Various special cases of the number theoretic transform such as the Fermat Number Transform or Mersenne Number Transform use a composite modulus (see e.g. Schönhage–Strassen algorithm).

For the number theoretic transform to work in a useful way when the modulus is composite (for the inverse algorithm, convolution etc. to work), it suffices that the modulus is chosen so that a primitive root of order n exists (where n is the transform length), and such that the multiplicative inverse of n exists. Note that if α is a primitive root of order n, then α is automatically invertible with inverse αn − 1.

## Properties

Most of the important attributes of the complex DFT, including the inverse transform, the convolution theorem, and most fast Fourier transform (FFT) algorithms, depend only on the property that the kernel of the transform is a primitive root of unity. These properties also hold, with identical proofs, over arbitrary fields. This analogy can be formalized by the field with one element, considering any field with a primitive nth root of unity as an algebra over the extension field $\mathbf{F}_{1^n}.$

In particular, the applicability of O(nlog n) fast Fourier transform algorithms to compute the NTT, combined with the convolution theorem, mean that the number-theoretic transform gives an efficient way to compute exact convolutions of integer sequences. While the complex DFT can perform the same task, it is susceptible to round-off error in finite-precision floating point arithmetic; the NTT has no round-off because it deals purely with fixed-size integers that can be exactly represented.

## Fast algorithms

For the implementation of a "fast" algorithm (similar to how FFT computes the DFT), it is often desirable that the transform length is also highly composite, e.g. a power of two. However, there are specialized fast Fourier transform algorithms for finite fields, such as Wang and Zhu's algorithm, which are efficient regardless of whether the transform length factors.

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