For a connected $n$-dimensional manifold without boundary,Choose a locally finite good cover by coordinate disks, each with a local orientation. Compactly supported classes involve only finitely many of these disks at a time. The compact-support Mayer–Vietoris sequence gives one generator $u_i$ per disk and overlap relations $u_i=\pm u_j$, with the sign determined by the two local orientations. Passing to the direct limit over finite unions handles a noncompact manifold; a finite good cover of the whole manifold need not exist.

The overlap graph is connected, so all generators reduce to one up to sign. If local orientations can be chosen coherently, every relation has positive sign and the result is $\mathbb{Z}$. An orientation-reversing loop imposes $u=-u$, giving $\mathbb{Z}/2$. This is also the top-degree case of Poincare duality with the orientation local system, using compactly supported cohomology on the cohomology side.

The compact set $K=\{0\}\cup \{1/n!:n\ge 1\}$ generates the additive group $\mathbb{Q}$, since every positive integer denominator divides some factorial. With its usual topology, $\mathbb{Q}$ is metrizable and is not locally compact. Nor can it have a nonzero Haar measure finite on compact sets: invariance gives equal masses to all singletons; a positive singleton mass makes $K$ have infinite mass, whereas zero singleton masses make the countable group have zero total mass.

// Target: analysis.bigb

Let finitely many arcs have centers ${\theta }_j$, half-lengths $0<{\delta }_j\le 1$ and $D={\sum }_j{\delta }_j$. Each summand has positive real part on the closed disk, and $\Phi (0)=1$. On the $j$th arc its contribution has real part at least $1/(4D)$. Thus the analytic logarithm $A=\log \Phi$ obeys $A(0)=0$, $|\mathrm{Im}A|<\pi /2$, and $\mathrm{Re}A\ge \log (1/(4D))$ throughout the arc union. Approximate $A$ by an analytic polynomial and modulate its bounded imaginary part; a partial Fourier sum isolates one half of the polynomial, producing a large value from a bounded function.

A metric space is totally bounded when, for every $\epsilon >0$, it is covered by finitely many open balls of radius $\epsilon$.

A metric space is totally bounded when, for every $\epsilon >0$, finitely many open balls of radius $\epsilon$ cover it. Equivalently it has a finite $\epsilon$-net at every positive scale.

Embed $\mathbb{N}$ into the compact product $\{0,1{\}}^{\mathcal{P}(\mathbb{N})}$ by $j(n{)}_S=1_S(n)$ and take its closure $X$. The Tychonoff theorem makes $X$ a compact Hausdorff space, while $j(\mathbb{N})$ is countable, dense and discrete in its subspace topology. Yet $X$ is not metrizable. Any convergent subsequence $j(n_k)$ of distinct points would have a convergent coordinate for $S=\{n_2,n_4,\dots \}$, whereas that coordinate alternates between zero and one. This contradicts sequential compactness of a compact metric space. Thus a dense metrizable subspace does not force a separable compact Hausdorff ambient space to be metrizable.

Let continuous maps $f,g:X\rightrightarrows Y$ between compact Hausdorff spaces have an underlying split coequalizer $q:Y\to Z$ with set maps $s,t$ satisfying $qs=1$, $ft=1$ and $gt=sq$. Its equivalence relation is the image of the closed subset $\{(x,x^{\prime })\in X^2:g(x)=g(x^{\prime })\}$ under $(f,f)$. One inclusion follows from $qf=qg$; for the other use $x=t(y)$ and $x^{\prime }=t(y^{\prime })$ whenever $q(y)=q(y^{\prime })$. This image is compact and hence closed in the Hausdorff product $Y^2$. The quotient is therefore compact Hausdorff, giving the preserved coequalizer required by the Beck monadicity theorem.

On a nonempty compact Hausdorff space, let continuous nonnegative functions decrease pointwise. Their suprema decrease to a finite number $L$. For every $t<L$, the sets $\{f_n\ge t\}$ are nonempty nested closed sets; compactness gives a common point with limit value at least $t$. Thus the supremum of the pointwise limit is $L$. The limit need not be continuous, and this conclusion does not claim uniform convergence as in the Dini theorem.

A nonempty closed subsemigroup $S$ of a compact group is a subgroup. For $x\in S$ and a neighbourhood $U$ of the identity, choose $V$ with $V^{-1}V\subseteq U$ and cover the compact group by finitely many translates of $V$. Two of sufficiently many powers $1,x,x^2,\dots$ lie in the same translate, so some positive power of $x$ belongs to $U$. Hence the identity is a limit of positive powers and lies in $S$. The preceding powers converge to $x^{-1}$, which also lies in $S$. Applied to the closure of a positive-word monoid, this proves that it equals the closed subgroup generated by its generators, without assuming they already include their inverses.

The Peter-Weyl theorem makes finite-dimensional unitary representations separate elements of a compact group. Intersecting their closed kernels gives the identity subgroup. If the descending chain condition holds for closed subgroups, choose finitely many representations whose kernel intersection is already trivial: otherwise repeatedly exclude a surviving nonidentity element to obtain an infinite strict chain. Their direct sum is a faithful representation into a finite-dimensional unitary group. Compactness makes its image closed, and the closed-subgroup theorem gives the original group a compatible Lie group structure.

Fix a real angle fraction $\alpha$, and repeatedly multiply by $g=e^{2\pi i\alpha }$ in the circle group. The generated cyclic subgroup is $H_{\alpha }=\{g^k:k\in \mathbb{Z}\}$. Equivalently, the angle fractions are the fractional parts of $k\alpha$, with one full turn identified with zero.

If $\alpha =p/q$ is a rational number in lowest terms, with $q>0$ and $p,q$ coprime, then $g^q=1$ and no smaller positive power is $1$. Hence $H_{\alpha }$ is a finite cyclic group of order $q$, consisting of equally spaced points. For $\alpha =2/7$, the successive angle fractions, starting at $k=0$, areThe subgroup is finite, so it is a closed subgroup of the circle group; it is also a discrete subgroup. It is not an open subgroup, since every ambient neighbourhood of any of its points contains an arc, rather than only these seven points.

If $\alpha$ is an irrational number, no nonzero integer $k$ satisfies $k\alpha \in \mathbb{Z}$, so the powers never repeat. The stronger statement is that $H_{\alpha }$ is a dense set in the circle group: every nonempty open arc contains a power $g^k$. Thus its closure is all of $\mathbb{T}$, while $H_{\alpha }$ itself is a countable proper subgroup and is not a closed subgroup or a discrete subgroup. See irrational rotation and everywhere interval frequency under an irrational rotation for the corresponding motion and its long-term frequencies.

For $\alpha =\sqrt{2}-1$, direct computation of the fractional parts of $k\alpha$ gives, to six decimal places,For example, $\{3\alpha \}=3\sqrt{2}-4\approx 0.242641$ and $\{5\alpha \}=5\sqrt{2}-7\approx 0.071068$. These calculations specify exactly which points the diagram displays.

This example shows why algebraic generation and closure must be distinguished: one irrational rotation generates a countable cyclic subgroup, yet its closure is the whole uncountable compact group.

On the circle group $\mathbb{T}$, normalized Haar measure is the pushforward measure of uniform angle on $[0,2\pi )$. For a continuous function $f:\mathbb{T}\to \mathbb{C}$, defineThe factor $1/(2\pi )$ makes the integral of the constant function $1$ equal to $1$. For $g=e^{i\varphi }$, a change of angle givesThe last equality uses the $2\pi$-periodic function $u\mapsto f(e^{iu})$. Since the circle group is an abelian group, this also verifies right invariance. An arc of angular length $L$ has normalized Haar measure $L/(2\pi )$. In particular, the arc $A=\{e^{i\theta }:\pi /6\le \theta \le \pi /2\}$ and its quarter-turn $iA$ both have measure $(\pi /2-\pi /6)/(2\pi )=1/6$.

The continuous unitary characters ${\chi }_n(e^{i\theta })=e^{in\theta }$, indexed by integers, give a fully computed family of integrals:Here $n$ must be an integer, so $z^n$ is well defined and $e^{2\pi in}=1$. Every nonconstant mode cancels exactly over a full turn. For example, the integral of $f(e^{i\theta })=3+2\cos \theta -\sin (2\theta )$ is $3$, and the integral of ${\cos }^2\theta =(1+\cos (2\theta ))/2$ is $1/2$. More generally,This is the orthogonality of complex exponentials underlying the Fourier basis and Fourier series: invariant averaging separates distinct frequencies and retains the constant component.

For nearby redshifts in a homogeneous expanding universe, the comoving radial distance increment is $c\delta z/H(z)$. Multiplying by the scale factor at the absorber's epoch gives the displayed proper distance in cosmology. The conversion neglects peculiar velocities; observed redshift differences can also contain such velocities and errors in the quasar emission redshift.

For pressureless homologous spherical collapse, let $\varphi$ be its cycloid parameter. The density contrast equation becomes ${\delta }_{\varphi \varphi }-\tan (\varphi /2){\delta }_{\varphi }-3\delta /(1+\cos \varphi )=0$. One solution is ${\delta }_g=\sin \varphi /(1+\cos \varphi {)}^2$. Reduction of order gives an independent solution that decays at collapse,For $ϵ=\pi -\varphi$, the integral is ${ϵ}^5/40+O({ϵ}^7)$, while ${\delta }_g\sim 4{ϵ}^{-3}$. Therefore ${\delta }_d\sim {ϵ}^2/10$. Since $R\sim R_0{ϵ}^2/4$, the displayed powers follow. The growing branch is unique up to scale and addition of the decaying branch; many mixed solutions grow, so it is not literally the only function with growth. Finite perturbations eventually leave the linear regime.

For a flat single-fluid background with constant barotropic equation of state, $w\ne -1/3$, adiabatic perturbations and zero anisotropic stress, $a\propto {\eta }^{2/(1+3w)}$ and $a^2\bar{\rho }\propto {\eta }^{-2}$. The scalar potential obeys ${\varphi }^{\prime \prime }+[6(1+w)/(1+3w)]{\varphi }^{\prime }/\eta -w{\nabla }^2\varphi =0$. Substituting the Poisson constraint $\Delta \propto {\eta }^2{\nabla }^2\varphi$ gives the displayed equation. The homogeneous mode and the degenerate vacuum fluid $w=-1$ require separate interpretation.

During matter domination in a flat universe, the growing comoving-gauge density contrast is proportional to $a\propto {\eta }^2$. Its Newtonian potential is constant by the Poisson constraint. The momentum constraint gives $v=-2\varphi /(3\mathcal{H})=-\eta \varphi /3$. Thus the peculiar velocity potential grows as $a^{1/2}$ in the convention where the peculiar velocity is $\nabla v$.

For adiabatic scalar perturbations without anisotropic stress, define $A=4\pi G{a}^2(\bar{\rho }+\bar{P})={\mathcal{H}}^2-{\mathcal{H}}^{\prime }$ and $c_a^2={\bar{P}}^{\prime }/{\bar{\rho }}^{\prime }$. In conformal Newtonian gauge, $\mathcal{R}=-\varphi -\mathcal{H}({\varphi }^{\prime }+\mathcal{H}\varphi )/A$. Background conservation gives $A^{\prime }/A=-\mathcal{H}(1+3c_a^2)$. Differentiating the expression for $\mathcal{R}$ and using the scalar-potential equation gives the displayed identity. For regular adiabatic modes with nonzero background enthalpy, evolution is suppressed on scales $k\ll \mathcal{H}$.

Two elements commute when their two products in opposite orders agree. A particular pair of matrices may commute even when matrix multiplication is not a commutative operation on the whole set of matrices.

Two elements $a,b$ of a commutative ring are associates if $a=ub$ for some unit $u$. This is an equivalence relation: reflexivity uses $1$, symmetry uses $u^{-1}$, and transitivity uses a product of units.

In $\mathbb{Z}$, the units are $1,-1$, so associates differ only by sign. In the polynomial ring $k[X]$ over a field, two nonzero polynomials are associates exactly when one is a nonzero scalar multiple of the other. Unique factorization identifies factors up to this unavoidable ambiguity. In an integral domain, nonzero elements generate the same principal ideal exactly when they are associates.

Functions on a finite group form a Hopf algebra under pointwise multiplication, with $(\Delta f)(g,h)=f(gh)$, $\epsilon (f)=f(1)$ and $(Sf)(g)=f(g^{-1})$. The coproduct is cocommutative exactly when the group is abelian, since delta functions distinguish distinct products $gh,hg$.

For prime $p$, a graph with $m>(p-1)n$ edges and maximum degree at most $2p-1$ contains a nonempty $p$-regular subgraph. Over ${\mathbb{F}}_p$, apply the Combinatorial Nullstellensatz on the Boolean edge-variable cube toIts full squarefree coefficient is nonzero because the first product has degree below $m$. A nonzero value cannot occur at the zero vector; at any other Boolean vector the second product vanishes, and the first forces all selected degrees to be multiples of $p$. The maximum-degree condition makes every positive selected degree exactly $p$.