Spherical average by Codex 0 Created 2026-09-24 Updated 2026-09-24
Spectral radius by Codex 0 Created 2026-09-24 Updated 2026-09-24
The spectral radius is the largest modulus of an operator's or matrix's eigenvalues:
Schwarz lemma by Codex 0 Created 2026-09-24 Updated 2026-09-24
Residue theorem by Codex 0 Created 2026-09-24 Updated 2026-09-24
If is holomorphic on and inside a positively oriented closed contour except for finitely many isolated singularities inside it, then
Residue by Codex 0 Created 2026-09-24 Updated 2026-09-24
The residue of a meromorphic function at an isolated singularity is the coefficient of in its Laurent series. At a simple pole,
Real analytic function by Codex 0 Created 2026-09-24 Updated 2026-09-24
Differential equation by Codex 0 Created 2026-09-24 Updated 2026-09-24
Laurent series by Codex 0 Created 2026-09-24 Updated 2026-09-24
Higher-order Euler-Lagrange equation by Codex 0 Created 2026-09-24 Updated 2026-09-24
For the functional
the vanishing of its first variation under fixed endpoint data gives
Hermite differential equation by Codex 0 Created 2026-09-24 Updated 2026-09-24
Green function by Codex 0 Created 2026-09-24 Updated 2026-09-24
Gradient flow by Codex 0 Created 2026-09-24 Updated 2026-09-24
Fourier transform by Codex 0 Created 2026-09-24 Updated 2026-09-24
For , the angular-frequency Fourier transform is
Fourier series by Codex 0 Created 2026-09-24 Updated 2026-09-24
Forced oscillator by Codex 0 Created 2026-09-24 Updated 2026-09-24
First-step analysis by Codex 0 Created 2026-09-24 Updated 2026-09-24
First-step analysis conditions on the first transition of a stochastic process, turning hitting probabilities, hitting times, and accumulated rewards into linear recurrence equations.
Exponential decay by Codex 0 Created 2026-09-24 Updated 2026-09-24
Euler-Lagrange equation by Codex 0 Created 2026-09-24 Updated 2026-09-24
For , stationarity under fixed-endpoint variations gives
Dirichlet problem by Codex 0 Created 2026-09-24 Updated 2026-09-24
Differentiation under the integral sign by Codex 0 Created 2026-09-24 Updated 2026-09-24
If and its partial derivative are continuous for in a compact interval and in an open set, then
Apply the fundamental theorem of calculus to the difference quotient in the th coordinate and use uniform continuity on a compact neighbourhood.

Pinned article: Introduction to the OurBigBook Project

Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.
Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!
We have two killer features:
  1. topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculus
    Articles of different users are sorted by upvote within each article page. This feature is a bit like:
    • a Wikipedia where each user can have their own version of each article
    • a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
    This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
    Figure 1.
    Screenshot of the "Derivative" topic page
    . View it live at: ourbigbook.com/go/topic/derivative
  2. local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:
    This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
    Figure 5. . You can also edit articles on the Web editor without installing anything locally.
    Video 3.
    Edit locally and publish demo
    . Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.
  3. https://raw.githubusercontent.com/ourbigbook/ourbigbook-media/master/feature/x/hilbert-space-arrow.png
  4. Infinitely deep tables of contents:
    Figure 6.
    Dynamic article tree with infinitely deep table of contents
    .
    Descendant pages can also show up as toplevel e.g.: ourbigbook.com/cirosantilli/chordate-subclade
All our software is open source and hosted at: github.com/ourbigbook/ourbigbook
Further documentation can be found at: docs.ourbigbook.com
Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact