The principal stretches are the eigenvalues of a stretch tensor, or equivalently the singular values of the deformation gradient. They measure local length ratios along the principal reference directions.

An $N$-dimensional conically stratified space has a closed filtration $\varnothing =X_{-1}\subseteq X_0\subseteq \cdots \subseteq X_N=X$, with each nonempty $X_j\setminus X_{j-1}$ a $j$-manifold, $X_{N-1}=X_{N-2}$, and dense top stratum. At an $s$-stratum the conical link is a compact $(N-s-1)$-dimensional stratified pseudomanifold. In dimension zero a compact link is a finite set. Connected links are an additional normality condition, not part of this definition.

A stratum is one manifold piece of a stratification, with locally constant dimension and prescribed conical normal neighbourhoods. Its codimension in a pure $N$-dimensional space is $N$ minus its dimension.

Suppose the variance is ${\sum }_i{v}_i/x_i$ and the sampling cost is ${\sum }_i{a}_i{x}_i\le b$, with positive costs, variance coefficients and budget. The Cauchy-Schwarz inequality gives $A^2\le ({\sum }_i{v}_i/x_i)({\sum }_i{a}_i{x}_i)\le b{\sum }_i{v}_i/x_i$. Equality forces $x_i$ proportional to $\sqrt{v_i/a_i}$ and a tight budget, proving the displayed unique optimum. The minimal variance is $A^2/b$. For the optimization Lagrangian $f-\lambda ({\sum }_i{a}_i{x}_i-b)$, the multiplier and budget derivative both equal $-A^2/b^2$. Therefore a small resource change $\delta b$ changes the optimum by $\lambda \delta b+O((\delta b{)}^2)$. This is a continuous allocation; integer sample-size restrictions define a separate optimization problem.

Instantaneous mixing efficiency is the fraction of instantaneous irreversible mechanical-energy loss that raises background potential energy, after subtracting internal-to-potential exchange. The ratio $\Gamma =M/ϵ$ is a different quantity, related by $\eta =\Gamma /(1+\Gamma )$. Replacing $M$ by buoyancy exchange $B$ additionally assumes negligible change of available potential energy.

For fixed wavevector with $k_z\ne 0$, fixed nonzero Alfvén frequency $a$, and $|\Omega |\to \infty$, the two roots in $X={\omega }^2$ of the stratified magneto-Coriolis wave polynomial have $X_f\sim 4{\Omega }^2{k}_z^2/K^2$ and the displayed slow root. This follows from the sum of roots being $4{\Omega }^2{k}_z^2/K^2+O(1)$ and their product being $a^2(K^2{a}^2-H{h}^2)/K^2$. The slow branch is oscillatory for $K^2{a}^2>H{h}^2$ and growing or decaying for the reversed inequality. At equality its frequency vanishes and the original stationary equations must be used.

A weak horizontal force $f^{\prime }(x)e^{imz}$, with $m>0$ and $\int f^{\prime }(x)dx=\epsilon$, admits many steady pressures differing by an arbitrary function of height. Switching the force on in initially motionless, unbounded, uniformly stratified fluid selects $p_{\infty }=[f(x)-\epsilon /2]e^{imz}$, up to a spatially constant pressure gauge. For horizontal wavenumber $q$, the transient frequency is $\Omega (q)=Nq/\sqrt{q^2+m^2}$. Its long-wave group velocity is $N/m$, and oppositely propagating outgoing fronts leave pressure limits $-\epsilon /2$ and $+\epsilon /2$. The limit is at fixed $x$ after the fronts have passed; dispersive tails need not vanish at any finite time.

For a continuous adapted process $X$ and $T=\inf \{s\ge 0:X_s\ge \lambda \}$,Continuity makes the supremum on $[0,t]$ attained and equals its supremum on this countable dense set. Each displayed event belongs to ${\mathcal{F}}_t$, so $T$ is a stopping time without requiring right-continuity of the filtration.

// Target: martingale.bigb

Place a normal Stokeslet below a plane and an equal, oppositely directed image at the reflected point above it. The normal velocity of their sum is odd across the plane and the tangential velocity is even. The plane therefore has zero normal velocity and zero normal derivative of tangential velocity, giving zero tangential stress. For a sphere of radius $a$ translating towards that plane at speed $U$ from distance $d\gg a$, the leading radial surface flow is $3aUrd/[2(r^2+d^2{)}^{3/2}]$, directed outward. The image's incident flow at the sphere changes its required force by relative order $a/d$, so the first correction to surface velocity at fixed $r/d$ is $O(U(a/d{)}^2)$; the isolated sphere's potential-dipole correction occurs only at third order.

A normal point force $F\mathbf{n}$ at height $d$ above a plane with a no-slip boundary condition is balanced by an image Stokeslet $-F\mathbf{n}$, a stresslet of tensor strength $S$, and a source dipole of moment $\mathbf{M}$ at the reflected point. Here the stresslet convention is $u_i=-S_{jk}{\partial }_k{G}_{ij}$, with $G$ the free-space Stokes tensor. Substitution on the plane cancels both normal and tangential velocity. The original and image forces cancel their $r^{-1}$ terms, while the image stresslet cancels the remaining $r^{-2}$ force-dipole term; the generic fixed-angle far field is $O(r^{-3})$.

For variations on a time scale $T$, the ratio of local inertia to viscous resistance is $L^2/(\nu T)$. Steady Stokes flow at each instant requires both $UL/\nu \ll 1$ and $L^2/(\nu T)\ll 1$. If only the first is small, retain local acceleration:For an oscillating sphere with angular frequency $\omega$, the second condition becomes $\omega a^2/\nu \ll 1$ on the radius scale $a$. The Unsteady Stokes equation is still linear, but needs initial data and has memory; its velocity field is not determined by instantaneous boundary velocities alone. See E. J. Hinch, Slow Viscous Flow, example sheet 1, question 1 for the distinction between steady and unsteady limits of an oscillating sphere.

Two plane internal gravity waves can each have zero Stokes drift but give a nonzero drift through interference. The displacement of one wave need not be perpendicular to the other wave's wave vector. For upward waves with wave vectors $(\kappa ,-\kappa )$ and $(-\kappa ,-\kappa )$ and vertical amplitudes $W$, the cross-wave drift is $(0,-2\kappa W^2\cos (2\kappa x)/\omega )$ with $\omega =N/\sqrt{2}$. It has zero spatially averaged vertical flux. It cannot on its own satisfy an impermeable oscillating boundary: the second-order Eulerian mean velocity supplies the necessary normal correction there.

For queue-class pairs $a$, external arrival rates ${\nu }_a$, and routing probabilities $p_{ab}$, the effective throughputs satisfy ${\lambda }_a={\nu }_a+{\sum }_b{\lambda }_b{p}_{ba}$. In an open network with routing spectral radius less than one, $\lambda =(I-P^T{)}^{-1}\nu$ is the unique nonnegative solution. Summing gives ${\sum }_a{\lambda }_a{p}_{a0}={\sum }_a{\nu }_a$, balancing total entrances and exits.

Connect multiclass single-server queues by Markovian routing. Each node must either have common exponential service parameters across its classes or use a symmetric service discipline. If the traffic equations for a multiclass queueing network give loads ${\rho }_j={\sum }_r{\lambda }_{jr}/{\mu }_{jr}<1$, its equilibrium word law is the product of the individual queue laws. Queue populations are independent at a fixed equilibrium time. A proof reverses routing with $p_{ba}^{\ast }={\lambda }_a{p}_{ab}/{\lambda }_b$, external arrivals with ${\nu }_a^{\ast }={\lambda }_a{p}_{a0}$, and exits with $p_{a0}^{\ast }={\nu }_a/{\lambda }_a$, and interchanges the insertion and service position rules. Weight ratios verify each reversed transition, and the traffic equations match total rates. This is a quasireversibility construction. A standard ordered-queue formulation is described in Walton's paper on multiclass queueing networks, Section 2.

A closed migration process is a continuous-time Markov chain of populations moving between colonies with a conserved total number of individuals. A colony containing $k$ individuals has departure rate ${\varphi }_j(k)$, with ${\varphi }_j(0)=0$, and each departure chooses its next colony using a fixed routing matrix. Its colony populations can represent stages of a service system, including an idle stage.

// Target: probability-and-statistics.bigb

The useful infinite-horizon path space for a stable queue isFor $\delta ,\sigma >0$, the workload functional $R(f)={\sup }_{t\ge 0}(\sigma f(t)-\delta t)$ is finite and continuous. Fix $f$, and choose $T\ge 1$ so that $|\sigma f(t)|\le \delta t/4$ for $t\ge T$. If $\Vert g-f{\Vert }_{\mathrm{sl}}<\delta /(4\sigma )$, then $\sigma g(t)-\delta t<0$ for $t\ge T$. Both suprema can thus be taken over $[0,T]$, whereUniform convergence on bounded time intervals alone does not give this continuity: a pulse escaping to later times can produce a large workload while tending locally to zero.

Local time records how much a stochastic process visits a level. Its normalization depends on the process: when the state space is a discrete space, it is occupation time at a state, while local time of a semimartingale is an occupation density weighted by quadratic variation. Brownian local time is a special case.

For an Itô process $d{X}_t=b_{t}dt+{\sigma }_{t}d{B}_t$ and a twice differentiable function $f$,The second-order term reflects the nonzero quadratic variation of Brownian motion.

A Prony brake measures shaft power using a resisting band: its force difference gives torque $\tau =r(F_1-F_2)$, and rotation gives $P=\tau \omega$, where $\omega$ is angular velocity. The Stirling engine experiment above uses this independent load measurement to distinguish shaft output from gas-cycle work.

For a spatially homogeneous minimally coupled scalar field with zero scalar potential, the stress-energy tensor has $\rho =P={\dot{\varphi }}^2/2$. Its field equation is $\ddot{\varphi }+3H\dot{\varphi }=0$, so $a^3\dot{\varphi }$ is constant and its energy density decreases as $a^{-6}$. Thus its homogeneous stress is precisely that of a stiff fluid. Spatial gradients would introduce directional stresses, so this equivalence requires homogeneity.