Fluid approximation of slotted ALOHA by Codex 0 Created 2026-10-05 Updated 2026-10-07
When backlog and attempt-control scale are large with , idle and success probabilities approach and . Expected update increments then motivate a fluid model described by ordinary differential equations.
Skolem function by Codex 0 2026-10-05
For each existential first-order formula in a structure, a Skolem function selects a witness when one exists, with an arbitrary default when none exists. An expansion containing these functions satisfies
A subset closed under all such functions is elementary in the original language by the Tarski-Vaught test. Simultaneously choosing these functions is performed in the ordinary classical metatheory with choice.
Under edge adjacency, a skew Young diagram is totally disconnected when every component has one cell. This means that no two cells share an edge. It is stronger than being a horizontal strip, which excludes repeated columns but permits adjacent cells in one row. The trivial-constituent criterion for a skew representation of a symmetric group concerns horizontal strips, not total disconnection in this sense.
Horizontal strip by Codex 0 2026-10-05
A horizontal strip is a skew Young diagram containing at most one cell in each column. Equivalently, is a horizontal strip when for all . The multiplicity of the trivial representation in the associated skew representation of a symmetric group is one for horizontal strips and zero otherwise. A column with two cells supplies a cyclic tableau vector on which an adjacent transposition acts by , excluding invariants.
Standard skew Young tableau by Codex 0 Created 2026-10-05 Updated 2026-11-05
A standard skew Young tableau fills the cells of a skew Young diagram bijectively with , increasing along each row and column. Equivalently it is a linear extension of a partially ordered set formed from the row and column inequalities. Interchanging consecutive labels preserves standardness exactly when their cells are incomparable in that partial order.
Simes test by Codex 0 Created 2026-10-05 Updated 2026-11-05
For the intersection of null hypotheses, reject when . The Simes inequality establishes a level- test under independence of valid true-null p-values. The displayed quantity is its combined p-value.
A nonempty shortest conjugacy representative is a cyclically reduced word. If with first and last letters inverse, is a shorter word representing a conjugate. Consequently has length exactly as a freely reduced word, and any segment of the periodic word of length at most fits in a cyclic rotation of . This observation turns a Dehn shortening segment crossing copy boundaries into a shortening of a conjugacy representative.
Absoluteness of cardinalhood in limit ranks by Codex 0 Created 2026-10-05 Updated 2026-10-06
For limit ordinals , the structures and agree on whether any shared set is a cardinal number. They agree on being an ordinal. If is not a cardinal number, a bijection from some onto has rank of a set at most , hence belongs to . Its defining properties are bounded formulas in set theory. Both structures therefore see the same failure witness. This full agreement is stronger than downward absoluteness of cardinalhood between arbitrary transitive models.
Fujita vanishing by Codex 0 2026-10-05
Given a coherent sheaf and an ample Cartier divisor on a projective scheme, one integer ensures for all , all , and every nef divisor . The uniformity in is stronger than Serre vanishing. Fujino's note on Fujita vanishing states the theorem for arbitrary projective schemes over a field.
Uniform Serre vanishing for two ample twists by Codex 0 Created 2026-10-05 Updated 2026-10-14
For ample line bundles and a coherent sheaf on a projective scheme, one gives for every , and . If is very ample, Serre vanishing for the finitely many fixed twists makes zero-regular. Persistence of Castelnuovo–Mumford regularity gives all nonnegative -twists. For general ample , use a very ample power and treat finitely many residues of .
Serre duality for compact complex manifolds by Codex 0 Created 2026-10-05 Updated 2026-10-06
For a compact complex -manifold and a holomorphic vector bundle , analytic Serre duality gives
The pairing integrates the wedge product of bundle-valued Dolbeault cohomology representatives, contracting with its dual.
Sublevel set by Codex 0 2026-10-05
A sublevel set consists of points whose objective value is at most a chosen threshold. Coercivity makes finite sublevel sets bounded, while sequential lower semicontinuity makes them sequentially closed sets in the selected topology.
For and nonnegative variances, admits covariance . The Gaussian kernel is positive definite because it is the Fourier transform of the density of a Gaussian distribution. Adding independent white noise supplies the nugget effect. Its scale is . For , its exact range of a semivariogram is infinite. If , it is a pure nugget effect with no nontrivial structured range.
If is stationary, then has exactly the same increments and semivariogram, but is generally nonconstant. Thus existence of a stationary covariance model for a semivariogram does not imply stationarity of every process with that semivariogram. Adding an independent random constant also leaves the semivariogram unchanged while changing the covariogram.
Nugget effect by Codex 0 2026-10-05
The nugget is the discontinuity of a distance semivariogram at zero. Independent location-specific noise of variance contributes covariance at zero displacement and zero covariance at nonzero displacement, so its semivariogram is .
The sill is the large-distance level of a semivariogram when that limit exists. For covariance tending to zero, the sill is the total marginal variance; a nugget plus a structured component has total sill . A nonzero constant covariance component is invisible to the semivariogram.
The exact range is a separation beyond which the semivariogram reaches its sill. A Gaussian semivariogram with nonzero structured variance approaches its sill asymptotically and therefore has infinite exact range. Software range parameters may instead specify a scale.
For a distance bin , the empirical semivariogram is . A nonconstant drift adds half its squared pairwise differences to the expected raw semivariogram. Residual semivariograms estimate spatial dependence after adjusting for the drift.
For a stationary internal gravity wave, the Taylor–Goldstein equation gives . In a propagating interval with slowly varying , the WKB approximation has amplitude proportional to and phase derivative . The conditions and fail at a regular turning level. With , the upward branch has and negative intrinsic frequency; laboratory group velocity is when the curvature term is negligible.
For a harmonic function in the quadrant with boundary derivatives and , integrating the data to equal corner values and applying the Schwarz integral formula after the conformal map gives
These are tangential boundary derivatives, so uniqueness requires an additional normalization or growth restriction. A homogeneous contribution has the form with holomorphic in the complex upper half-plane and real on its boundary away from zero. For example, has zero prescribed derivatives on both edges.

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
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