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The Levi-Civita connection is the unique torsion-free connection compatible with the metric; its coefficients are the Christoffel symbols formed from first metric derivatives.

For $W_{\mu \nu }$,

Parallel transport along a path $\gamma$ sends an initial fiber vector to the endpoint of the unique horizontal lift through that vector. Reversing the path gives the inverse linear map.

A tensor of type $(r,s)$ has $r$ contravariant and $s$ covariant indices. Taking a covariant derivative adds one covariant index, while an outer tensor product adds the corresponding contravariant and covariant index counts.

The covariant divergence contracts a covariant-derivative index with a contravariant tensor index. For a mixed rank-two tensor it is ${\nabla }_{\mu }{T}^{\mu }{}_{\nu }$.

For a metric $\rho$, the first Wasserstein distance is

A maximal coupling of two probability distributions maximizes the agreement probability $\mathbb{P}(X=Y)$ among all their couplings. Equivalently, it minimizes $\mathbb{P}(X\ne Y)$. The coupling characterization of total variation distance identifies the minimum mismatch with the total variation distance of the two marginal laws. The term maximal concerns agreement, rather than the size of a correlation coefficient or closeness in a geometric metric.

On a finite set, retain all common probability mass at equal pairs and match the excesses elsewhere. The explicit four-outcome example in coupling characterization of total variation distance realizes this construction and checks its marginal distributions. A maximal coupling need not be unique: different joint arrangements of the residual mass can have the same optimal mismatch.

Cantor's diagonal argument shows that the set of all infinite binary sequences is an uncountable set. Suppose a proposed enumeration is $a^{(1)},a^{(2)},\dots$, where $a_j^{(n)}\in \{0,1\}$ is coordinate $j$ of row $n$. Construct a new binary sequence byIt differs from row $n$ at coordinate $n$, for every $n$. It is consequently absent from the entire proposed list, regardless of how that list was chosen. The failure of an enumeration of all binary sequences follows from this precise point of disagreement, rather than from a claim that the list looks too small.

For the displayed window, the first six diagonal bits are $1,0,0,0,1,1$, so the new sequence starts $0,1,1,1,0,0$. The drawing shows only finitely many rows, but the rule defines every coordinate and excludes every row of an infinite list. This distinguishes the argument from the Cantor pairing function: that construction lists all pairs of finite indices, whereas this one concerns an entire infinite sequence as each object to be listed. Finite binary strings form a countable set; all infinite binary sequences do not.

The connection with power sets is exact: a subset $A\subseteq \mathbb{N}$ corresponds to its indicator function, with a one in position $n$ exactly when $n\in A$. Changing the diagonal membership bit is the same mechanism as the Cantor's theorem construction $D=\{x:x\notin f(x)\}$. This reusable method turns a purported complete list into an explicit omitted object.

In his paper ^[ref], presented in 1891 and printed in the 1890–91 volume in 1892, Cantor uses infinite sequences of two symbols for the diagonal construction and then gives the broader comparison of cardinalities. The binary notation here is a modern presentation of that construction.

The Cantor pairing function is an explicit bijection between ${\mathbb{N}}^2$ and $\mathbb{N}$, with the natural numbers numbered from zero:If $d=x+y$, there are $1+2+\cdots +d=d(d+1)/2$ pairs on earlier diagonals, and the pair $(x,y)$ is at offset $y$ along diagonal $d$. The formula therefore gives exactly the enumeration above. For example, $\pi (2,3)=5\cdot 6/2+3=18$.

For the inverse at an index $k$, find the unique $d$ satisfying $d(d+1)/2\le k<(d+1)(d+2)/2$, then put $y=k-d(d+1)/2$ and $x=d-y$. At $k=18$, these are $d=5$, $y=3$, and $x=2$. Thus both the encoding and decoding are explicit. The bijection is useful for assigning a single natural number to a pair of indices, and iterating it does the same for fixed-length tuples.

A countable union of countable sets is countable, using a diagonal enumeration.

With singly ionized helium and helium mass fraction $Y_p$, charge neutrality gives

Chemical equilibrium for $A^{+}+e^{-}\leftrightarrow A^0+\gamma$ givesafter neglecting the ion--atom mass difference in the translational prefactor.

A constant barotropic equation of state has $P=w\rho$. The cases $w=0$ and $w=1/3$ describe pressureless matter and radiation respectively.

A particle species thermally decouples when its interaction rate falls below the Hubble parameter. Its collisionless relativistic distribution subsequently redshifts with temperature proportional to $a^{-1}$, while annihilations among still-coupled species can heat the remaining plasma.

The effective number of relativistic entropy degrees of freedom is defined byA relativistic bosonic degree of freedom contributes one, while a relativistic fermionic degree of freedom contributes $7/8$ when all species share a temperature.

A cosmological fluid with constant equation-of-state parameter $w$ has $P=w\rho$ and $\rho \propto a^{-3(1+w)}$.

Before recombination, frequent Thomson scattering couples photons, electrons, and baryons into an acoustic fluid.

For Fourier wavevector $\mathbf{k}$ and photon direction $\hat{\mathbf{p}}$, the temperature perturbation is expanded as

Thomson scattering of a local radiation quadrupole generates linear cosmic microwave background polarization. Its acoustic phase follows the photon velocity and is shifted by one quarter-period relative to the monopole temperature oscillation.

For a canonical scalar field with potential $V(\varphi )$, slow roll reduces the field and Friedmann equations to