ChebPy is a Python implementation of Chebfun, bringing the power of Chebyshev polynomial approximations to Python. It allows you to work with functions as first-class objects, performing operations like differentiation, integration, and root-finding with machine precision accuracy.

Table of Contents

• ✨ Features
• 📥 Installation
• 🛠️ Development
• 🚀 Quick Start
• 📖 Documentation
• 📄 License
• 👥 Contributing

✨ Features

Work with functions as easily as numbers

• 🔢 Function Approximation: Automatic Chebyshev polynomial approximation of smooth functions
• 🌊 Periodic Functions: Fourier-based approximation via trigfun for smooth periodic functions
• ♾️ Infinite Intervals: Functions on $[a, \infty)$, $(-\infty, b]$ or the full real line via CompactFun
• 📍 Endpoint Singularities: Resolve $\sqrt{x}$, $\sqrt{x(1-x)}$ and similar branch-type endpoints to spectral accuracy
• 📐 Calculus Operations: Differentiation, integration, and root-finding with machine precision
• 📊 Plotting: Beautiful function visualizations with matplotlib integration
• 🧮 Arithmetic: Add, subtract, multiply, and compose functions naturally
• 🎯 Adaptive: Automatically determines optimal polynomial degree for given tolerance
• 🔁 Convolution: Convolve two Chebfuns to produce a new function
• 📏 Quasimatrices: Continuous linear algebra via QR, SVD, and least-squares
• 🎲 Gaussian Process Regression: GP posteriors returned as Chebfun objects
• 🔗 Interoperability: Works seamlessly with NumPy and SciPy ecosystems

📥 Installation

Using pip (recommended)

pip install chebpy

From source (development)

git clone https://github.com/chebpy/chebpy.git
cd chebpy
pip install -e .

Note: Use -e flag for editable installation during development

🛠️ Development

For contributors and advanced users

ChebPy uses modern Python development tools for a smooth developer experience:

# 📦 Install development dependencies
make install

# 🧪 Run tests with coverage
make test

# ✨ Format and lint code
make fmt
make lint

# 📓 Start interactive notebooks
make marimo

# 🔍 View test coverage report
make coverage

Development Tools

• Testing: pytest with coverage reporting
• Formatting: ruff for code formatting and linting
• Notebooks: marimo for interactive development
• Task Management: Taskfile for build automation

Quick Start

This figure was generated with the following simple ChebPy code:

import numpy as np
from chebpy import chebfun

# Create functions as chebfuns on interval [0, 10]
f = chebfun(lambda x: np.sin(x**2) + np.sin(x)**2, [0, 10])
g = chebfun(lambda x: np.exp(-(x-5)**2/10), [0, 10])

# Find intersection points
roots = (f - g).roots()

# Plot both functions and mark intersections
ax = f.plot(label='f(x) = sin(x²) + sin²(x)')
g.plot(ax=ax, label='g(x) = exp(-(x-5)²/10)')
ax.plot(roots, f(roots), 'ro', markersize=8, label='Intersections')
ax.legend()
ax.grid(True, alpha=0.3)

More Examples

# Differentiation and integration
f = chebfun(lambda x: np.exp(x) * np.sin(x), [-1, 1])
df_dx = f.diff()          # Derivative
integral = f.sum()        # Definite integral

# Root finding
g = chebfun(lambda x: x**3 - 2*x - 5, [-3, 3])
roots = g.roots()         # All roots in the domain

Convolution

Convolve two functions to produce a new Chebfun on the summed domain:

from chebpy import chebfun
import numpy as np

f = chebfun(lambda x: np.exp(-x**2), [-1, 1])
g = chebfun(lambda x: np.where(np.abs(x) < 0.5, 1.0, 0.0), [-1, 1])

h = f.conv(g)        # h(x) = ∫ f(t) g(x−t) dt, a Chebfun on [−2, 2]
h.plot()

Quasimatrices

Stack functions as columns of an ∞×n matrix and use continuous linear algebra — QR, SVD, least-squares:

from chebpy import Quasimatrix, chebfun

x = chebfun("x")
A = Quasimatrix([1, x, x**2, x**3, x**4, x**5])

Q, R = A.qr()             # QR factorisation → Legendre polynomials
U, S, V = A.svd()         # Singular value decomposition

f = chebfun(lambda t: np.exp(t) * np.sin(6 * t), [-1, 1])
c = A.solve(f)            # Least-squares polynomial fit
f_approx = A @ c          # Reconstruct as a Chebfun

Gaussian Process Regression

Fit a GP to scattered data and get the posterior mean and variance back as Chebfuns — ready for differentiation, integration, and root-finding:

from chebpy import gpr
import numpy as np

rng = np.random.default_rng(1)
x_obs = np.sort(-2 + 4 * rng.random(10))
y_obs = np.sin(np.exp(x_obs))

f_mean, f_var = gpr(x_obs, y_obs, domain=[-2, 2])

f_mean.plot()                     # Posterior mean (a Chebfun)
extrema = f_mean.diff().roots()   # Local extrema via calculus
integral = f_mean.sum()           # Definite integral

Periodic Functions

Use trigfun for smooth periodic functions — the same API as chebfun, but backed by a Fourier (Trigtech) representation that is far more compact for periodic targets:

from chebpy import trigfun
import numpy as np

f = trigfun(lambda x: np.exp(np.sin(np.pi * x)), [-1, 1])
len(f)            # number of Fourier modes
f.diff()          # spectral differentiation in Fourier space
f.sum()           # ≈ 2 · I₀(1)

The gpr interface accepts trig=True for a periodic GP posterior, also returned as a Trigtech-backed Chebfun.

Infinite Intervals

Pass np.inf or -np.inf as a domain endpoint to construct a Chebfun on a (semi-)infinite interval. Pieces with infinite endpoints are automatically built as CompactFun objects: a Chebyshev expansion on the discovered numerical-support window, with optional non-zero tail constants (tail_left, tail_right) recovered automatically for sigmoid-like inputs (tanh, logistic, …).

from chebpy import chebfun
import numpy as np

# Doubly-infinite Gaussian — sum is √π
h = chebfun(lambda x: np.exp(-x**2), [-np.inf, np.inf])
h.sum()                           # ≈ √π

# Sigmoid-like: tail constants are detected automatically
t = chebfun(np.tanh, [-np.inf, np.inf])
t.funs[0].tail_left, t.funs[0].tail_right    # (-1.0, 1.0)
t(1e10)                                       # 1.0

# Mixed: finite breakpoints with infinite endpoints
p = chebfun(lambda x: np.exp(-x**2), [-np.inf, -2.0, 0.0, 3.0, np.inf])
[type(piece).__name__ for piece in p.funs]
# ['CompactFun', 'Bndfun', 'Bndfun', 'CompactFun']

Endpoint Singularities

Functions with branch-type singularities at one or both endpoints — such as $\sqrt{x}$ on $[0, 1]$ — cannot be resolved by ordinary Chebyshev interpolation. Pass sing="left", "right", or "both" to switch the boundary pieces to Singfun, which uses an exponential clustering map to recover spectral accuracy.

The map $m: [-1, 1] \to [a, b]$ is the Adcock–Richardson slit-strip construction (Adcock & Richardson, SIAM J. Numer. Anal. 52(4), 1887–1912, 2014; doi:10.1137/130920460; arXiv:1305.2643). For sing="left", with $s = L(t-1)/2$,

$$ m(t) = a + (b-a),\frac{\alpha}{\pi},\log(1 + e^{\pi(s + \gamma)/\alpha}), $$

where $\gamma = (\alpha/\pi),\log(e^{\pi/\alpha} - 1)$ is chosen so the smooth endpoint $b$ is hit exactly, and $m'(\pm 1) \to 0$ super-exponentially at the clustered endpoint. A MapParams(L, alpha) object tunes truncation and clustering; defaults work for the canonical $\sqrt{\cdot}$ cases:

from chebpy import chebfun
import numpy as np

# Plain Bndfun fails to converge for sqrt; Singfun resolves it to machine
# precision in ~150 coefficients.
f = chebfun(np.sqrt, [0.0, 1.0], sing="left")
f.sum()                       # 2/3, to machine precision

# Two-sided singularity on the same domain
g = chebfun(lambda x: np.sqrt(x * (1 - x)), [0.0, 1.0], sing="both")
g.sum()                       # pi/8, to machine precision

Mixed-piece arithmetic (Singfun + Bndfun, etc.) is preserved, and restrict automatically falls back to Bndfun on subintervals that exclude the clustered endpoint. conv and diff on Singfun pieces are not yet supported.

Documentation

• 🚀 Codespaces: Try ChebPy in your browser
• 📚 Documentation: Full documentation, user guide, and API reference

📄 License

ChebPy is licensed under the 3-Clause BSD License.

📜 See the full license in the LICENSE.rst file.

👥 Contributing

We welcome contributions! 🎉

Whether you're fixing bugs, adding features, or improving documentation, your help makes ChebPy better for everyone.

Quick Start for Contributors

1. 🍴 Fork the repository
2. 🌿 Create your feature branch

   git checkout -b feature/amazing-feature
   

3. ✨ Make your changes and add tests
4. 🧪 Test your changes

   make test
   

5. 📝 Commit your changes

   git commit -m 'Add amazing feature'
   

6. 🚀 Push to your branch

   git push origin feature/amazing-feature
   

7. 🎯 Open a Pull Request

Resources

• 📋 Contributing Guide
• 🤝 Code of Conduct
• 🐛 Issue Tracker

Acknowledgments 🙏

ChebPy stands on the shoulders of the Chebfun project led by Nick Trefethen and the Chebfun development team at the University of Oxford. The mathematical design, algorithmic ideas, and naming conventions in this library are direct adaptations of their work — most notably:

• The original MATLAB Chebfun system (github.com/chebfun/chebfun).
• L. N. Trefethen, Approximation Theory and Approximation Practice, SIAM, 2013 (extended edition 2019).
• T. A. Driscoll, N. Hale, and L. N. Trefethen (eds.), Chebfun Guide, Pafnuty Publications, 2014.

We are grateful for their decades of open scholarship, which made this Python port possible. Any errors in translation or adaptation are ours alone.

📜 See the History of the Chebfun Project page for a fuller timeline, key contributors, and foundational publications.

Project tooling:

• Jebel-Quant/rhiza for standardised CI/CD templates and project tooling