Isotropic stellar mass loss changes the central gravitational parameter $\mu (t)$ while preserving the central direction of the force, provided the escaping material causes no recoil or drag on the orbiting body. The relative equation is $\ddot{\mathbf{r}}=-\mu (t)\mathbf{r}/r^3$. Specific angular momentum remains exactly constant; specific orbital energy obeys $\dot{C}=-\dot{\mu }/r$. Orbital expansion is adiabatic only when the mass-loss law varies slowly on the orbital period.

Orbit crossing occurs when two geometrical orbital paths intersect. It permits collisions or close encounters when bodies also reach the intersection at compatible times; crossing alone does not imply a collision. For nearly circular coplanar nested orbits, a useful local test follows by differentiating their radial positions at fixed longitude.

Surface albedo is the fraction of incident solar energy flux reflected by a surface, averaged over the specified spectrum and illumination geometry. The locally absorbed fraction is $1-\alpha$. It is distinct from geometric albedo, which measures brightness in a particular viewing configuration.

Ocean heat content relative to a reference temperature $T_f$ is an integral through the water column. For a uniform column of depth $H$, $Q={\rho }_w{c}_{w}H(T-T_f)$ per horizontal area. Melting ice already at its fusion temperature requires its latent heat; warming cold ice first requires additional sensible heat. A surface temperature alone does not determine the column integral.

The solar constant is solar irradiance on a plane normal to incoming sunlight at a specified reference distance from the Sun, conventionally the mean Earth-Sun distance. Local horizontal irradiance includes solar geometry and atmospheric transmission.

An astroid is the four-cusped plane curve parametrized by $x=a{\cos }^{3}t$, $y=a{\sin }^{3}t$ for $a>0$. It appears as the relative trajectory in the astroid motion of a harmonic oscillator in a rotating frame. The parametrization has zero first derivative at the cusps while remaining smooth as a function of its parameter.

If a finite measure on ${\mathbb{R}}^n$ has square-integrable Fourier transform, the Plancherel theorem gives an $L^2$ inverse transform $g$. Pairing with every Schwartz function and using Fourier inversion identifies the measure with $gdx$. This establishes absolute continuity, rather than presuming it. For a positive measure, $g\ge 0$ and $g\in L^1$ as well. Multiplication by $f\in L^{\infty }(\mu )$ yields $\Vert \widehat{fd\mu }{\Vert }_2\le \Vert \hat{\mu }{\Vert }_2\Vert f{\Vert }_{L^{\infty }(\mu )}$.

Parametrize a positively oriented closed plane curve at constant arc length speed on $[0,2\pi ]$, and let $c_n$ be its complex-position Fourier coefficients. Green's theorem and Parseval's identity give $A=\pi {\sum }_{n}n|c_n{|}^2$ and $L^2/(4{\pi }^2)={\sum }_n{n}^2|c_n{|}^2$. Since $n\le n^2$ for integers, the planar isoperimetric inequality follows. Equality leaves only the constant and positive first harmonic, a translated circle.

Such a field embedding pulls the p-adic absolute value back to a finite place of a number field above $p$. It extends to an embedding of the corresponding completion, whose image is a finite extension of the P-adic numbers.

Suppose two constant data intensities differ by $d$, have areas $a$ and $A-a$, and share an internal boundary of length $L$. Keeping the boundary costs $\beta L$ with zero fidelity; merging the two regions costs $a(A-a)d^2/A$ after fitting their common mean. The displayed inequality makes splitting cheaper among these two candidate geometries. This comparison does not prove either geometry is globally optimal among all partitions.

The physical Thomson scattering rate equals the inverse photon mean scattering time. Comparing it with the Hubble parameter gives a local approximate decoupling criterion. A more precise last-scattering temperature is obtained from optical depth and the visibility maximum; these conventions need not give exactly the same temperature.

A quantum phase transition is a change of the ground state as a nonthermal control parameter is varied at zero temperature. Pressure, interaction strength or an applied magnetic field can tune the competing contributions to the Hamiltonian. The transition is driven by changes in the quantum ground state, whereas a thermal phase transition is driven by thermal occupation and entropy.

The Transverse-field Ising model illustrates the competition between aligned-spin order and a field favoring a different spin direction. A continuous transition has a quantum critical point, with a closing excitation gap and growing correlation scales; a quantum phase transition can also be first order. Experiments operate at finite temperature, where the nearby quantum-critical regime can reveal the influence of the zero-temperature transition without placing the sample exactly at zero.

Freezing is a liquid-to-solid phase transition. A moving phase boundary releases latent heat; its rate is constrained by heat transport and the Stefan condition. In a solution the liquidus depends on composition, so being below the pure solvent's melting temperature does not imply that the solution must freeze.

Melting is a solid-to-liquid phase transition that absorbs latent heat. It can be driven by heating, changing phase pressures, or adding solute that causes freezing-point depression. The ice layer between freshwater and cold brine illustrates melting into a cold liquid through dilution and a composition-dependent liquidus.

An iterative expansion of long-range connectivity functions that separates an approximately random-walk contribution from corrections caused by loops and intersections. In suitable high-dimensional percolation models it controls these corrections and proves mean-field percolation exponents. It also applies to other models, including self-avoiding walks; its use always requires model-specific estimates.

For critical site percolation on the triangular lattice, scaling-limit crossing probabilities transform naturally under conformal maps between planar domains. This rigorously established property supports descriptions of interfaces by SLE and calculations of planar percolation critical exponents. Transferring it to a different lattice or to bond percolation requires a separate universality argument.

The identity follows because the identity matrix and the three Pauli matrices form an orthogonal basis of two-by-two matrices under the trace inner product. It rewrites a spin-vector bilinear square as a scalar four-fermion expression, with the Grassmann signs supplied when fields are reordered. This is the algebra behind a spin-vector Hubbard–Stratonovich decoupling.

Three light quarks have a six-dimensional single-quark spin-flavour space ${\mathbb{C}}^3\otimes {\mathbb{C}}^2$. A symmetric spatial ground state and the antisymmetric three-quark colour singlet require the symmetric cube of that space. Its dimension is $\left(\genfrac{}{}{0ex}{}{8}{3}\right)=56$, and restriction to flavour $SU(3)$ and spin $SU(2)$ gives a baryon decuplet of spin $3/2$ and a baryon octet of spin $1/2$. The two mixed-permutation multiplicity spaces pair once in the symmetric sector, explaining why the two flavour-octet copies in the unconstrained tensor cube do not represent two distinct ground-state octets.

A Hamiltonian path is a path in a graph that visits every graph vertex exactly once. Removing any edge from a Hamilton cycle produces a Hamiltonian path. For a simple graph with at least three vertices, a Hamiltonian path closes to a Hamilton cycle exactly when its endpoints are joined by an edge.

For any positive integer $c$, a homogeneous rational matrix is partition regular over $c\mathbb{N}$ if and only if it is partition regular over $\mathbb{N}$. Pull a colouring of $c\mathbb{N}$ back along $n\mapsto cn$ and scale the resulting solution. Conversely, restrict a colouring of all positive integers to $c\mathbb{N}$. The argument depends on homogeneity and does not automatically extend to inhomogeneous equations or prescribed distinctness conditions.