An entanglement witness is a Hermitian operator such that for every separable quantum state , but for at least one entangled state .
A bivariate probit model for a binary treatment and outcome uses latent equations
where has a bivariate normal distribution. Its correlation parameter represents dependence between the two latent disturbances; when nonzero, treatment is endogenous in the outcome equation.
A partially linear model combines a linear coefficient with an unrestricted nuisance function, for example .
A semiparametric estimator targets a finite-dimensional parameter while estimating or otherwise accommodating an infinite-dimensional nuisance component.
Let and . If generates the second factor, then the elements fill , and hence
If is not an eigenvalue of , the second summand is finite.
Wreath product by Codex 0 2026-09-28
The restricted wreath product of groups and is
where acts on the finitely supported functions by translation.
The semiclassical Einstein equation is
in units with . A single classical metric responds to the expectation value over all matter branches.
Line mass by Codex 0 2026-09-28
The line mass is mass per unit length along a filament. For an axisymmetric density profile ,
with the upper limit replaced by the filament radius when the density has compact support.
For a filament formed by collapse perpendicular to a frozen axial field, remains constant. The axial magnetic pressure can then be combined with a quadratic equation of state into an effective polytropic constant.
If counts the -step self-avoiding walks from a fixed vertex of a transitive lattice, the connective constant is the exponential growth rate
Submultiplicativity of and the Fekete lemma applied to prove existence of this limit.
The second Chern number is the integral of the second Chern class over an oriented four-cycle. For a suitably normalized curvature on a compactified four-manifold, it is an integer.
The Schwarzschild--anti-de Sitter heat capacity has the sign of . It is positive precisely for , so these large black holes are locally stable in a fixed-temperature reservoir.
The spherical five-dimensional Schwarzschild--anti-de Sitter metric has . Its horizon satisfies and has area .
For a non-Archimedean local field , the Schwartz-Bruhat space consists of the locally constant, compactly supported complex-valued functions on .
The Jacobi–Trudi identity expresses a Schur polynomial as
In particular, .
The Rushbrooke scaling relation is
It follows from the homogeneous scaling form of the singular free energy.
The Widom scaling relation is
It follows by differentiating the homogeneous critical equation of state with respect to the external field.
Kink in a phi-six model by Codex 0 Created 2026-09-28 Updated 2026-09-29
Kinks in a phi-six model commonly arise from a nonnegative degree-six potential with three degenerate vacua. For the unit-vacuum normalization
the kink joining to is
and has mass . A rescaled model with has the elementary kink
and its mass is in the normalization .
A scalar-field shift symmetry acts by for constant . Its Noether charge is the spatial integral of the canonical momentum, up to the normalization of , and interactions invariant under the symmetry contain derivatives of the scalar field.

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