Associate to each prime $p\equiv 3(\mathrm{mod}4)$ the class of $(-1,p{)}_{\mathbb{Q}}$. These classes have order two, since quaternion conjugation identifies the algebra with its opposite. The product rule $[(-1,a)][(-1,b)]=[(-1,ab)]$ follows by factoring their tensor product into the commuting subalgebras generated by $i_1,j_1{j}_2$ and by $i_1{i}_2,j_2$; the second is the split algebra $(1,b)$. Every nonempty finite product of distinct such primes has an odd valuation at one of them. The quaternion norm obstruction at primes congruent to three modulo four therefore makes its class nonzero, proving independence. Euclid's construction $4p_1\cdots p_m-1$ proves there are infinitely many such primes.

For a number field $K$, localization of the Brauer group is injective. Its image is exactly the finite-support tuples of local classes whose local Brauer invariants sum to zero. The non-Archimedean summands have invariant group $\mathbb{Q}/\mathbb{Z}$, real summands have $\{0,1/2\}$, and complex summands are zero. For a cyclic character $\chi$ and parameter $a$, the invariant of the local cyclic algebra is $\chi ({\mathrm{Art}}_{K_v}(a))$. Thus the zero-sum condition on principal parameters is the additive form of Artin reciprocity.

The Brauer class of a central simple algebra is its equivalence class under adjoining matrix-algebra factors. Two algebras have the same class exactly when their central division-algebra representatives are isomorphic. The zero class consists of split matrix algebras; the inverse of $[A]$ is the class of the opposite algebra. For a cyclic algebra $(L/K,\sigma ,a)$, the class is zero exactly when $a$ is a field norm from $L$.

Assume four spacetime dimensions, $\Phi \ne 0$, constant $\omega$, no scalar potential and matter independent of $\Phi$. In the stated action convention, scalar variation gives $\Phi R+\omega {\Phi }^{-1}(\nabla \Phi {)}^2-2\omega \square \Phi =0$. The trace of the metric tensor equation gives $\Phi R+\omega {\Phi }^{-1}(\nabla \Phi {)}^2+3\square \Phi =8\pi GT$. Their subtraction proves the displayed wave equation with $T=g^{ab}{T}_{ab}$. The degenerate coupling of Brans-Dicke theory needs separate treatment before dividing by its coefficient.

Take two copies of each direction $(1,0),(0,1),(1,1),(1,-1)$. The resulting box spline has total polynomial degree six and $C^4$ continuity: there are eight directions and a rank-deficient remaining set has at most two, so the removal number is six. With a centred origin its support is the zonotopeIts centred binary symbol isThis symbol factors as $A(z,w)A(zw,z/w)$ with $A=(2+z+z^{-1})(2+w+w^{-1})/8$, giving an efficient quincunx subdivision implementation. The box spline criterion is also described in the primary research paper 4–8 Subdivision by Velho and Zorin.

This box spline is the density of the image of the uniform unit four-cube under the indicated direction matrix. Its polynomial pieces have total degree $4-2=2$. Removing three directions is necessary to leave a nonspanning set, giving $C^1$ continuity. Its knot arrangement is a criss-cross triangulation of a square lattice. Its refinement mask is $\frac{1}{4}(1+z)(1+w)(1+zw)(1+z/w)$, up to the monomial that fixes the origin.

The unit-direction univariate box spline is the density of the sum of $m$ independent uniform unit-interval coordinates. It is the order-$m$ Cardinal B-spline: its closed support is $[0,m]$, it is strictly positive on the interior, and its polynomial degree is $m-1$. For $m\ge 2$ it is $C^{m-2}$, generally not $C^{m-1}$. The recurrence $B_m(x)={\int }_0^1{B}_{m-1}(x-u)du$ proves its support and positivity. Starting with a half-open unit interval, integrating the translate partition recursively proves ${\sum }_{j\in \mathbb{Z}}{B}_m(x-j)=1$. Orthogonal projection onto the unit diagonal rescales support width from $m$ to $\sqrt{m}$.

A transformed-response regression estimates the conditional mean $m$ on the transformed scale. Inverting that mean does not generally estimate the original-scale mean, because nonlinear transformations do not commute with expected values. For a smooth inverse $g$ and centered transformed error of variance ${\sigma }^2$, a second-order Taylor expansion gives the displayed local correction. For $g(z)=z^q$ at positive $m$, it is $m^q+\frac{1}{2}q(q-1)m^{q-2}{\sigma }^2$. The variance is the response's residual variance, not the sampling variance of the estimated mean. This approximation requires errors small enough that the inverse is meaningful over the relevant range; it is not an exact moment formula for arbitrary transformed noise.

A bounding volume hierarchy traversal visits only nodes whose ancestors passed a conservative enclosure test. It can use depth-first search; its time complexity depends on how many nodes pass, not just the height of a binary tree.

// Target: optics.bigb

Let $S_n={\sum }_{j=1}^n{X}_j$, where the increments are nonconstant independent and identically distributed random variables with zero expected value and $|X_j|\le c$. Then the random walk oscillates between arbitrarily large positive and negative levels almost surely. For $T_a=\inf \{n:S_n\ge a\}$, the stopped martingale $a+c-S_{n\wedge T_a}$ is nonnegative. The martingale convergence theorem implies that $S_n$ converges finitely on $\{T_a=\infty \}$. However, some $\delta >0$ has $\mathbb{P}(|X_1|>\delta )>0$, and the Borel-Cantelli lemmas imply $|X_n|>\delta$ infinitely often. Hence finite convergence is impossible. Apply this to every positive integer $a$ and to $-S_n$.

The bounded increments also imply infinitely many visits to $[-c/2,c/2]$: a path cannot cross from above $c/2$ to below $-c/2$ without entering this interval. Identical distribution cannot simply be dropped. For independent symmetric signs ${\epsilon }_j$, the increments $X_j=c{4}^{-(j-1)}{\epsilon }_j$ satisfy $|S_n|\ge 2c/3$ for every $n$, even though they are nonzero, independent, symmetric and uniformly bounded.

If $X_1,\dots ,X_n$ are independent random variables and $f$ has bounded differences constants $c_i$, thenand the same bound holds for the lower tail.

A meromorphic function on the unit disc has bounded characteristic if and only if it has this representation. Its pole Blaschke product $B$ makes $g=Bf$ analytic with bounded positive logarithmic means. The least harmonic majorant by expanding disk lifts gives $H\ge {\log }^{+}|g|$. A harmonic conjugate gives an analytic $A$ with $\mathrm{Re}A=H$, so $u=g{e}^{-A}$ and $v=B{e}^{-A}$ are bounded. Conversely the Nevanlinna first main theorem bounds the characteristic of $u/v$ by that of $u$ and $1/v$, hence by a constant.

For $0<\kappa <4$ and $x>0$, stop the simple chordal SLE trace on first entering the radius-$r$ half-disc about $x$, with $r<x/4$. A crosscut from its tip to $x$ separates the positive-side boundary arc containing $x/2$ from infinity. Harmonic measure comparison with the half-disc bounds the length of that arc's mapped interval, $g_t(x/2)-{\xi }_t$, by a universal constant times $r$. This centered boundary image is a positive Bessel process of dimension $1+4/\kappa >2$ and has a strictly positive all-time minimum. Hence the full trace stays a positive distance from each fixed nonzero real point. Reflection gives the negative-side version.

For a drop with internal viscosity $\lambda \mu$, exterior viscosity $\mu$, continuous velocity and a decaying exterior Stokes flow, use the drop-outward normal $n$ and curvature $\kappa ={\nabla }_s\cdot n$, twice the mean curvature. Constant surface tension gives $({\sigma }^{+}-{\sigma }^{-})n=\gamma \kappa n$. Multiply the interior boundary representation by $\lambda$ and add it to the exterior one: the traction difference yields the single-layer term shown, while the viscosity-independent stress kernel yields $(\lambda -1)D[u]$. Here $S,D$ denote the kernels in boundary integral representation of Stokes flow. For equal viscosities the double layer vanishes.

For a homogeneous ideal gas in $d$ dimensions with $A,\alpha >0$, the density of states scales as ${ϵ}^{d/\alpha -1}$. At limiting chemical potential zero, the Bose-Einstein distribution is proportional to $1/ϵ$ near zero energy. The excited-particle integral is finite exactly when $d/\alpha >1$. Thus positive-temperature condensation at fixed nonzero density is possible precisely for $0<\alpha <d$. A nonconserved particle number, as for ordinary equilibrium photons, removes the fixed-number saturation mechanism even when the integral is finite.

For real $h\in C_c^{\infty }(D)$, the displayed real compactly supported contrast has Fourier transform $\hat{V}(\mathbf{\kappa })=[|\mathbf{\kappa }{|}^4-4({\mathbf{k}}_i\cdot \mathbf{\kappa }{)}^2]\hat{h}(\mathbf{\kappa })$. This vanishes on the shifted Ewald sphere because $|\mathbf{\kappa }{|}^2+2{\mathbf{k}}_i\cdot \mathbf{\kappa }=0$ there. Thus nonzero contrasts can have zero fixed-frequency, fixed-incidence Born approximation data. A small amplitude preserves weak contrast and a positive refractive index.

For a topological space, ${\Sigma }_1^0$ is the class of open sets and ${\Pi }_{\alpha }^0$ consists of complements of ${\Sigma }_{\alpha }^0$ sets. For countable $\alpha >1$, ${\Sigma }_{\alpha }^0$ consists of countable unions of sets in ${\bigcup }_{\beta <\alpha }{\Pi }_{\beta }^0$. These countable levels contain every Borel set. Continuous inverse images preserve each level, by induction using openness, complementation and countable unions.

A bounded normal operator admits a normal square root even when no continuous scalar square root exists on its Banach algebra spectrum. Choose a bounded Borel branch $s(z)=|z{|}^{1/2}\exp (i\mathrm{Arg}(z)/2)$, with $s(0)=0$. The Borel functional calculus for a normal operator gives $S=s(T)$; multiplicativity gives $S^2=T$ and preservation of the adjoint gives $S^{\ast }S=S{S}^{\ast }=|s{|}^2(T)$. Unlike a positive square root, this square root need not be unique.

For a bounded Borel measurable function $f$ and the projection-valued measure $E$ of a normal operator, the displayed set is the spectrum of an element of $f(T)$. Outside it, a bounded reciprocal away from a spectral-null set supplies the inverse. Inside it, unit vectors supported by the indicated arbitrarily small spectral sets make $f(T)-\lambda$ fail to be bounded below. The kernel of the calculus consists of functions satisfying $E(|f|>\epsilon )=0$ for every $\epsilon >0$, and its norm is the corresponding essential supremum. Ordinary pointwise range is insufficient: for multiplication by $t$ on $L^2[0,1]$, $1_{\{0\}}(T)=0$ even though the function takes the value $1$.

An unordered nonparametric bootstrap sample from $n$ distinct observations is specified by its $n$ nonnegative multiplicities. Stars and bars gives $\left(\genfrac{}{}{0ex}{}{2n-1}{n}\right)$ such vectors. They are not equiprobable: a vector has probability $n!/(n^n{\prod }_i{N}_i!)$, because this counts its ordered sequences of sampled indices.