Posthumous Chebyshev Polynomials

Posted on 12 February 2025 by John

Two families of orthogonal polynomials are named after Chebyshev because he explored their properties. These are prosaically named Chebyshev polynomials of the first and second kind.

I recently learned there are Chebyshev polynomials of the third and fourth kind as well. You might call these posthumous Chebyshev polynomials. They were not developed by Mr. Chebyshev, but they bear a family resemblance to the polynomials he did develop.

The four kinds of Chebyshev polynomials may be defined in order as follows.

$\begin{align*} T_n(\cos\theta) &= \cos n\theta \\ U_n(\cos\theta) &= \frac{\sin (n+1)\theta}{\sin \theta} \\ V_n(\cos\theta) &= \frac{\cos \left(n+\frac{1}{2}\right)\theta}{\cos \frac{1}{2}\theta} \\ W_n(\cos\theta) &= \frac{\sin \left(n+\frac{1}{2}\right)\theta}{\sin \frac{1}{2}\theta} \\ \end{align*}$

It’s not obvious that these definitions even make sense, but in each case the right hand side can be expanded into a sum of powers of cos θ, i.e. a polynomial in cos θ. [1]

All four kinds of Chebyshev polynomials satisfy the same recurrence relation

$P_n(x) = 2x\,P_{n-1}(x) - P_{n-2}(x)$

for n ≥ 2 and P₀ = 1 but with different values of P₁, namely x, 2x, 2x − 1, and 2x + 1 respectively [2].

Plots

We can implement Chebyshev polynomials of the third kind using the recurrence relation above.

def V(n, x):
    if n == 0: return 1
    if n == 1: return 2*x - 1
    return 2*x*V(n-1, x) - V(n-2, x)

Here is a plot of V_n(x) for n = 0, 1, 2, 3, 4.

The code for implementing Chebyshev polynomials of the fourth kind is the same, except the middle line becomes

    if n == 1: return 2*x + 1

Here is the corresponding plot.

Square roots

The Chebyshev polynomials of the first and third kind, and polynomials of the second and fourth kind, are related as follows:

$\begin{align*} V_n(x)&=\sqrt\frac{2}{1+x}T_{2n+1}\left(\sqrt\frac{x+1}{2}\right) \\ W_n(x)&=U_{2n}\left(\sqrt\frac{x+1}{2}\right) \end{align*}$

To see that the expressions on the right hand side really are polynomials, note that Chebyshev polynomials of the first and second kinds are odd for odd orders and even for even orders [3]. This means that in the first equation, every term in T_{2n + 1} has a factor of √(1 + x) that is canceled out by the 1/√(1 + x) term up front. In the second equation, there are only even powers of the radical term so all the radicals go away.

You could take the pair of equations above as the definition of Chebyshev polynomials of the third and fourth kind, but the similarity between these polynomials and the original Chebyshev polynomials is more apparent in the definition above using sines and cosines.

The square roots hint at how these polynomials first came up in applications. According to [2], Chebyshev polynomials of the third and fourth kind

have been called “airfoil polynomials”, since they are appropriate for approximating the single square root singularities that occur at the sharp end of an airfoil.

Dirichlet kernel

There’s an interesting connection between Chebyshev polynomials of the fourth kind and Fourier series.

The right hand side of the definition of W_n is known in Fourier analysis as D_n, the Dirichlet kernel of order n.

$D_n(\theta) = \frac{\sin \left(n+\frac{1}{2}\right)\theta}{\sin \frac{1}{2}\theta}$

The nth order Fourier series approximation of f, i.e. the sum of terms −n through n in the Fourier series for f is the convolution of f with D_n, times 2π.

$(D_n * f)(\theta) = 2\pi \sum_{k=-n}^n \hat{f}(k) \exp(ik\theta)$

Note that D_n(θ) is a function of θ, not of x. The equation W_n(cos θ) = D_n(θ) defines W_n(x) where x = cos θ. To put it another way, D_n(θ) is not a polynomial, but it can be expanded into a polynomial in cos θ.

[1] Each function on the right hand side is an even function, which implies it’s at least plausible that each can be written as powers of cos θ. In fact you can apply multiple angle trig identities to work out the polynomials in cos θ.

[2] J.C. Mason and G.H. Elliott. Near-minimax complex approximation by four kinds of Chebyshev polynomial expansion. Journal of Computational and Applied Mathematics 46 (1993) 291–300

[3] This is not true of Chebyshev polynomials of the third and fourth kind. To see this note that V₁(x) = 2x − 1, and W₁(x) = 2x + 1, neither of which is an odd function.

Trigonometric interpolation

Posted on 5 November 2024 by John

Suppose you want to interpolate a set of data points with a combination of sines and cosines.

One way to approach this problem would be to set up a system of equations for the coefficients of the sines and cosines. If you have N data points, you will get a system of N equations in N unknowns. The system will have a unique solution, though this is not obvious a priori.

Another approach would be to use the discrete Fourier transform (DFT). This is the approach that would commonly be used in practice. It’s even further from obvious a priori that this would work, but it does. (The DFT is so often computed using the FFT algorithm that the transform is often referred to by the algorithm name. If you’d like, mentally substitute FFT for DFT in the rest of the post.)

There are multiple ways to motivate the DFT, and the way suggested by the name is to derive the DFT as a discrete approximation to the (continuous) Fourier transform. Why should a discrete approximation to an integral transform also solve an interpolation problem? This doesn’t sound inevitable, or even plausible, but it is the case.

Another way to motivate the DFT is as the least-squares solution to fitting a sum of sines and cosines to a set of points. Since this is phrased as an optimization problem rather than an interpolation problem, it is clear that it will have a solution. However, it is not clear that the error in the optimal fit will in fact be zero. Furthermore, the equation for the coefficients in the solution is the same as the equation for the DFT. You can find a derivation in [1].

Example

Let’s take the vector [3, 1, 4, 1, 5, 9] and find trig functions that pass through these points. We can use the FFT as implemented in Python’s SciPy library to find a set of complex exponentials that pass through the points.

    from scipy.fft import fft
    from numpy import exp, array, pi, round

    x = array([3, 1, 4, 1, 5, 9])
    y = fft(x)

    N = len(x)
    z = [sum([exp(2j*pi*k*n/N)*y[k] for k in range(N)])/N for n in range(N)]

Aside from rounding errors on the order of 10⁻¹⁵ the vector z equals the vector x.

Turning the expression for z above into a mathematical expression, we have

f(z) = y₀ + y₁ exp(nπi/3) + y₂ exp(2nπi/3) + y₃ exp(nπi) + y₄ exp(4nπi/3) + y₅ exp(5nπi/3)

where the y‘s come from the FFT above.

To find sines and cosines we need to use Euler’s formula

exp(iθ) = cos(θ) + i sin(θ)

Because started with real data x, there will be symmetries in the FFT components x that simplify the reduction of the complex function f to a real-valued function g using sines and cosines; some of the components will be conjugate and so the complex parts cancel out.

6 g(x) = y₀ + (y₁ + y₅) cos(πx/3) + (y₂ + y₄) cos(2πx/3) + y₃ cos(πx)
+ i (y₁ − y₅) sin(πx/3) + (y₂ − y₄) cos(2πx/3)

and so

g(x) = 3.833 + 0.8333 cos(πx/3) − 1.833 cos(2πx/3) + 0.1666 cos(πx)
− 2.5981 sin(πx/3) − 2.0207 cos(2πx/3)

Here’s a plot that verifies that g(x) passes through the specified points.

[1] William L. Briggs and Van Emden Henson. The DFT: An Owner’s Manual for the Discrete Fourier Transform. SIAM 1995.

Sawtooth waves

Posted on 23 October 2024 by John

I woke up around 3:00 this morning to some sort of alarm outside. It did not sound like a car alarm; it sounded like a sawtooth wave. The pattern was like a few Morse code O’s. Not SOS, or I would have gotten up to see if anyone needed help. Just O’s.

A sawtooth wave takes its name from the shape of its waveform: it looks like the edge of a saw. It also sounds a little jagged.

Sawtooth waves have come up several times here. For one thing, they have rich harmonics. Because the wave form is discontinuous, the Fourier coefficients decay to zero slowly. I wrote about that here. The post is about square waves and triangular waves, but sawtooth waves are very similar.

Here’s a post oscillators with a sawtooth forcing function.

I took sawtooth functions in a different direction in this post that started with an exercise from Knuth’s TAOCP. This led me down a rabbit hole on replicative functions and multiplication theorems in different contexts.

If I remember correctly the sound used for red alterts in Star Trek TOS started with a sawtooth wave. Early synthesizers had sawtooth generators because, as mentioned above, these waves are rich in overtones and can be manipulated to create interesting sounds such as the red alert sound.

The Clausen function

Posted on 1 July 2024 by John

I ran across the Clausen function the other day, and when I saw a plot of the function my first thought was that it looks sorta like a sawtooth wave.

Plot of Clausen function Cl_2

I wondered whether it also sounds like a sawtooth wave. More on that shortly.

The Clausen function can be defined in terms of its Fourier series:

$\text{Cl}_2(x) = \sum_{k=1}^\infty \frac{\sin(kx)}{k^2}$

The function commonly known as the Clausen function is one of a family of functions, hence the subscript 2. The Clausen functions for all non-negative integers n are defined by replacing 2 with n on both sides of the defining equation.

The Fourier coefficients decay quadratically, as do those of a triangle wave or sawtooth wave, as discussed here. This implies the function Cl₂(x) cannot have a continuous derivative. In fact, the derivative of Cl₂(x) is infinite at 0. This follows quickly from the integral representation of the function.

$\text{Cl}_2(x)=-\int_0^x\log \left|2\sin\frac{t}{2} \right|\, dt$

The fundamental theorem of calculus shows that the derivative

$\text{Cl}'_2(x)=-\log \left|2\sin\frac{x}{2} \right|$

blows up at 0.

How does it sound?

What does it sound like if we create music with Clausen waves rather than sine waves? I initially thought it sounded harsh, but that turned out to be an artifact of how I’d make the audio file. A reader emailed me a better recording using the first few notes of a famous hymn. It’s a much more pleasant sound than I had expected.

ein_feste_burg.wav

Connecting the FFT and quadratic reciprocity

Posted on 3 February 2024 by John

Some readers will look at the title of this post and think “Ah yes, the FFT. I use it all the time. But what is this quadratic reciprocity?”

Others will look at the same title and think “Gauss called the quadratic reciprocity theorem the jewel in the crown of mathematics. But what is this FFT thing? I think I remember an engineer saying something about that.”

Gauss proved a theorem that relates quadratic reciprocity and the FFT. For distinct odd primes p and q, the following equation holds.

$\left(\frac{p}{q}\right) \left(\frac{q}{p}\right) = \frac{\text{Tr} {\cal F}_{pq}}{ \text{Tr} {\cal F}_p\, \text{Tr} {\cal F}_q}$

I’ll spend the rest of this post unpacking this equation.

Legendre symbols

The expressions on the left are not fractions but rather Legendre symbols. The parentheses are not for grouping but are part of the symbol.

The Legendre symbol

$\left(\frac{a}{r}\right)$

is defined to be 0 if a is a multiple of r, 1 if a has a square root mod r, and −1 otherwise.

Fourier transforms

The Discrete Fourier Transform (DFT) of a vector of length n multiplies the vector by the n by n Fourier matrix F_p whose j, k entry is equal to exp(2πi jk / n). The Fast Fourier Transform (FFT) is a way to compute the DFT more quickly than directly multiplying by the Fourier matrix. Since the DFT is nearly always computed using the FFT algorithm, the DFT is commonly referred to as the FFT.

Matrix trace

The trace of a matrix is the sum of the elements along the main diagonal. So the trace of the Fourier matrix of size n is

$\text{Tr} {\cal F}_n = \sum_{j=1}^n \exp(2\pi ij^2/n)$

Numerical illustration

The quadratic reciprocity theorem, also due to Gauss, is usually stated as

$\left(\frac{p}{q}\right) \left(\frac{q}{p}\right) = (-1)^{\frac{p-1}{2}\frac{q-1}{2}}$

We can illustrate the theorem at the top of the page numerically with the following Python code, using the quadratic reciprocity theorem to evaluate the product of the Legendre symbols.

from numpy import exp, pi

tr = lambda p: sum(exp(2j*pi*k**2/p) for k in range(1, p+1))
p, q = 59, 17
print( tr(p*q)/(tr(p)*tr(q)) )
print( (-1)**((p-1)*(q-1)/4) )

The first print statement produces (0.9999999999998136-1.4048176871018313e-13j) due to some loss of precision due to floating point calculations, but this is essentially 1, which is what the second print statement produces.

If we change q to 19, both statements print −1 (after rounding the first result).

Quadratic Gauss sum

We can quickly prove

$\left(\frac{p}{q}\right) \left(\frac{q}{p}\right) = \frac{\text{Tr} {\cal F}_{pq}}{ \text{Tr} {\cal F}_p\, \text{Tr} {\cal F}_q}$

if we assume the quadratic reciprocity theorem and the following equation for the trace of the Fourier matrix.

$\text{Tr} {\cal F}_n = \sum_{j=1}^n \exp(2\pi ij^2/n) = \left\{ \begin{array}{ll} \sqrt{n} & \mbox{if } n \equiv 1 \bmod{4} \\ 0 & \mbox{if } n \equiv 2 \bmod{4} \\ i\sqrt{n} & \mbox{if } n \equiv 3 \bmod{4} \\ (1+i)\sqrt{n} & \mbox{if } n \equiv 0 \bmod{4} \\ \end{array} \right.$

This proof is historically backward. It assumes quadratic reciprocity, but Gauss proved quadratic reciprocity by first proving the equation we’re trying to prove. He then obtained the expression on the right hand side of the quadratic reciprocity theorem using the equation above for the trace of the Fourier matrix.

The trace of the Fourier matrix is now called a quadratic Gauss sum. It’s a special case of more general sums that Gauss studied, motivated by his exploration of quadratic reciprocity.

Incidentally, Gauss gave many proofs of the quadratic reciprocity theorem. I don’t know where the proof outlined hear fits into the sequence of proofs he developed.

Eigenvectors of the DFT matrix

Posted on 3 September 2023 by John

When is the discrete Fourier transform of a vector proportional to the original vector? And when that happens, what is the proportionality constant?

In more formal language, what can we say about the eigenvectors and eigenvalues of the DFT matrix?

Setup

I mentioned in the previous post that Mathematica’s default convention for defining the DFT has mathematical advantages. One of these is that it makes the DFT an isometry, that is, taking the DFT of a vector does not change its norm. We will use Mathematica’s convention here because that will simplify the discussion. Under this convention, the DFT matrix of size N is the square matrix whose (j, k) entry is

ω^jk / √N

where ω = exp(-2π i/N) and the indices j and k run from 0 to N − 1.

Eigenvalues

Using the definition above, if you take the discrete Fourier transform of a vector four times, you end up back where you started. With other conventions, taking the DFT four times takes you to a vector that is proportional to the original vector, but not the same.

It’s easy to see what the eigenvalues of the DFT are. If transforming a vector multiplies it by λ, then λ⁴ = 1. So λ = ±1 or ±i. This answers the second question at the top of the post: if the DFT of a vector is proportional to the original vector, the proportionality constant must be a fourth root of 1.

Eigenvectors

The eigenvectors of the DFT, however, are not nearly so simple.

Suppose N = 4k for some k > 1 (which it nearly always is in practice). I would expect by symmetry that the eigenspaces of 1, −1, i and −i would each have dimension k, but that’s not quite right.

In [1] the authors proved that the eigenspaces associated with 1, −1, i and −i have dimension k+1, k, k−1, and k respectively.

This seems strange to me in two ways. First, I’d expect all the spaces to have the same dimension. Second, if the spaces did not have the same dimension, I’d expect 1 and −1 to differ, not i and −i. Usually when you see i and −i together like this, they’re symmetric. But the span of the eigenvectors associated with i has dimension one less than the dimension of the span of the eigenvectors associated with −i. I don’t see why this should be. I’ve downloaded [1] but haven’t read it yet.

[1] J. H. McClellan; T. W. Parks (1972). “Eigenvalues and eigenvectors of the discrete Fourier transformation”. IEEE Transactions on Audio and Electroacoustics. 20 (1): 66–74.

DFT conventions: NumPy vs Mathematica

Posted on 3 September 2023 by John

Just as there are multiple conventions for defining the Fourier transform, there are multiple conventions for defining the discrete Fourier transform (DFT), better known as the fast Fourier transform (FFT). [1]

This post will look at two DFT conventions, one used in Python’s NumPy library, and one used in Mathematica. There are more conventions in use, but this post will just look at these two.

In some sense the differences between conventions are trivial, but trivial doesn’t mean unimportant [1]. If you don’t know that there are multiple conventions, you could be quite puzzled when the output of a FFT doesn’t match your expectations.

NumPy definition

NumPy’s fft and related functions define the discrete Fourier transform of a sequence a₀, a₁, …, a_N−1 to be the sequence A₀, A₁, …, A_N−1 given by

$A_k = \sum_{m=0}^{N-1} a_m \exp(-2\pi i mk/N)$

Mathematica definition

Mathematica’s Fourier function defines the discrete Fourier transform of a sequence u₁, u₂, …, u_N to be the sequence v₁, v₂, …, v_N given by

$v_k = \frac{1}{\sqrt{N}} \sum_{m=1}^{N} u_m \exp\big(-2\pi i (m-1)(k-1)/N\big)$

This is the default definition in Mathematica, but not the only possibility. More on that below in the discussion of compatibility.

Motivation

Python arrays are indexed from 0 while Mathematica arrays are indexed starting from 1. This is why the inputs and outputs are numbered as they are.

Subtracting 1 from the m and k indices makes the two definitions visually less similar, but the terms in the two summations are the same. The only difference between the two implementations is the scaling factor in front of the sum.

Why does Mathematica divide the sum by √N while NumPy does not? As is often the case when there are differing conventions for defining the same thing, the differences are a result of which theorems you want to simplify. Mathematica complicates the definition of the DFT slightly, but in exchange makes the DFT and its inverse more symmetric.

The choice of scaling factor is consistent with the user bases of the two languages. Python skews more toward engineering and applied math, while Mathematica skews more toward pure math. In light of this, the choices made by Python and Mathematica seem inevitable.

Compatibility

Like Mathematica’s continuous Fourier transform function FourierTransform, its discrete Fourier transform function Fourier takes an optional FourierParameters argument for compatibility with other conventions. Setting the a parameter to 1 eliminates the √N term and produces a result consistent with NumPy.

There are more variations in DFT definitions. For example, some definitions of the DFT do not have a negative sign inside the exponential. Mathematica can accommodate this by setting b to −1 in thel FourierParameters argument. There are other possibilities too. In some implementations, for example, the 0 frequency DC term is in the middle rather than at the beginning.

[1] The FFT is an algorithm for computing the DFT, but the transform itself is often called the FFT.

[2] In classical education, the trivium consisted of grammar, logic, and rhetoric. The original meaning of “trivial” is closer to “foundational” than to “insignificant.”

DFT mandalas

Posted on 2 September 2023 by John

Math books often use some illustration from the book contents as cover art. When they do, there’s often some mystery to the cover art, and a sense of accomplishment when you get far enough into the book to understand the significance of the cover. (See examples here.)

William L. Briggs and Van Emden Henson wrote such a book, The DFT: An Owner’s Manual for the Discrete Fourier Transform. The cover features the following image.

At first glance, this image might look like a complete graph, one with an edge from every node along the circle to every other node. But that’s not it. For one thing, there’s only one line that goes through the center of the circle. And when you look closer it’s not as symmetric as it may have seemed at first.

Here’s the same image plotted in blue, except this time I reduced the alpha channel of the lines. Making the lines less opaque makes it possible to see that some lines are drawn more often than others, the darker lines being the ones that have been traced more than once.

What does this drawing represent? The explanation is given in the last exercise at the end of the first chapter.

The cover and frontmatter of this book display several polygonal, mandala-shaped figures which were generated using the DFT.

The exercise goes into some detail and invites the reader to reproduce versions of the cover figure with N = 4 or N = 8 nodes around the circle. For n from 1 to N, take the DFT (discrete Fourier transform) of the nth standard basis vector e_n and draw lines connecting the components of the DFT. These components are

F_k = exp(-2πink/N) / N

for k = 1 to N.

The cover also has smaller figures which correspond to the same sort of image for other values of N. For example, here is the figure for N = 8.

Circulant matrices, eigenvectors, and the FFT

Posted on 12 May 2023 by John

A circulant matrix is a square matrix in which each row is a rotation of the previous row. This post will illustrate a connection between circulant matrices and the FFT (Fast Fourier Transform).

Circulant matrices

Color in the first row however you want. Then move the last element to the front to make the next row. Repeat this process until the matrix is full.

The NumPy function roll will do this rotation for us. Its first argument is the row to rotate, and the second argument is the number of rotations to do. So the following Python code generates a random circulant matrix of size N.

    import numpy as np

    np.random.seed(20230512)
    N = 5
    row = np.random.random(N)
    M = np.array([np.roll(row, i) for i in range(N)])

Here’s the matrix M with entries truncated to 3 decimal places to make it easier to read.

    [[0.556 0.440 0.083 0.460 0.909]
     [0.909 0.556 0.440 0.083 0.460]
     [0.460 0.909 0.556 0.440 0.083]
     [0.083 0.460 0.909 0.556 0.440]
     [0.440 0.083 0.460 0.909 0.556]]

Fast Fourier Transform

The Fast Fourier Transform is a linear transformation and so it can be represented by a matrix [1]. This the N by N matrix whose (j, k) entry is ω^jk where ω = exp(-2πi/N), with j and k running from 0 to N – 1. [2]

Each element of the FFT matrix corresponds to a rotation, so you could visualize the matrix using clocks in each entry or by a cycle of colors. A few years ago I created a visualization using both clock faces and colors:

Eigenvectors and Python code

Here’s a surprising property of circulant matrices: the eigenvectors of a circulant matrix depend only on the size of the matrix, not on the elements of the matrix. Furthermore, these eigenvectors are the columns of the FFT matrix. The eigenvalues depend on the matrix entries, but the eigenvectors do not.

Said another way, when you multiply a circulant matrix by a column of the FFT matrix of the same size, this column will be stretched but not rotated. The amount of stretching depends on the particular circulant matrix.

Here is Python code to illustrate this.

    for i in range(N):
        ω = np.exp(-2j*np.pi/N)
        col1 = np.array([ω**(i*j) for j in range(N)])
        col2 = np.matmul(M, col1)
        print(col1/col2)

In this code col1 is a column of the FFT matrix, and col2 is the image of the column when multiplied by the circulant matrix M. The print statement shows that the ratios of each elements are the same in each position. This ratio is the eigenvalue associated with each eigenvector. If you were to generate a new random circulant matrix, the ratios would change, but the input and output vectors would still be proportional.

Notes

[1] Technically this is the discrete Fourier transform (DFT). The FFT is an algorithm for computing the DFT. Because the DFT is always computed using the FFT, the transformation itself is usually referred to as the FFT.

[2] Conventions vary, so you may see the FFT matrix written differently.

Fixed points of the Fourier transform

Posted on 17 March 2023 by John

This previous post looked at the hyperbolic secant distribution. This distribution has density

$f_H(x) = \frac{1}{2} \text{sech} \left(\frac{\pi x}{2} \right)$

and characteristic function sech(t). It’s curious that the density and characteristic function are so similar.

The characteristic function is essentially the Fourier transform of the density function, so this says that the hyperbolic secant function, properly scaled, is a fixed point of the Fourier transform. I’ve long known that the normal density is its own Fourier transform, but only recently learned that the same is true of the hyperbolic secant.

Hermite functions

The Hermite functions are also fixed points of the Fourier transform, or rather eigenfuctions of the Fourier transform. The eigenvalues are 1, i, -1, and i. When the eigenvalues are 1, we have fixed points.

There are two conventions for defining the Hermite functions, and multiple conventions for defining the Fourier transform, so the truth of the preceding paragraph depends on the conventions used.

For this post, we will define the Fourier transform of a function f to be

$\hat{f}(\omega) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^\infty \exp(-i \omega x)\, f(x)\, dx$

Then the Fourier transform of exp(-x²/2) is the same function. Since the Fourier transform is linear, this means the same holds for the density of the standard normal distribution.

We will define the Hermite polynomials by

$H_n(x) = (-1)^n e^{x^2}\frac{d^n}{dx^n}e^{-x^2}$

using the so-called physics convention. H_n is an nth degree polynomial.

The Hermite functions ψ_n(x) are the Hermite polynomials multiplied by exp(-x²/2). That is,

$\psi_n(x) = H_n(x) \exp(-x^2/2)$

With these definitions, the Fourier transform of ψ_n(x) equals (-i)ⁿ ψ_n(x). So when n is a multiple of 4, the Fourier transform of ψ_n(x) is ψ_n(x).

[The definition Hermite functions above omits a complicated constant term that depends on n but not on x. So our Hermite functions are proportional to the standard Hermite functions. But proportionality constants don’t matter when you’re looking for eigenfunctions or fixed points.]

Hyperbolic secant

Using the definition of Fourier transform above, the function sech(√(π/2) x) is its own Fourier transform.

This is surprising because the Hermite functions form a basis for L²(ℝ), and all have tails on the order of exp(-x²), but the hyperbolic secant has tails like exp(-x). Each Hermite function eventually decays like exp(-x²), but this happens later as n increases, so an infinite sum of Hermite functions can have thicker tails than any particular Hermite function.

Fourier analysis

Posthumous Chebyshev Polynomials

Plots

Square roots

Dirichlet kernel

Related posts

Trigonometric interpolation

Example

Related posts

Sawtooth waves

The Clausen function

How does it sound?

Related posts

Connecting the FFT and quadratic reciprocity

Legendre symbols

Fourier transforms

Matrix trace

Numerical illustration

Quadratic Gauss sum

Related posts

Eigenvectors of the DFT matrix

Setup

Eigenvalues

Eigenvectors

DFT conventions: NumPy vs Mathematica

NumPy definition

Mathematica definition

Motivation

Compatibility

DFT mandalas

Related posts

Circulant matrices, eigenvectors, and the FFT

Circulant matrices

Fast Fourier Transform

Eigenvectors and Python code

Notes

Fixed points of the Fourier transform

Hermite functions

Hyperbolic secant

Related posts