Semiheavy water contains one ordinary hydrogen nucleus and one deuterium nucleus. Dilute semiheavy water in heavy water provides relatively isolated O–H groups for infrared spectroscopy.

Heavy water replaces both ordinary hydrogen nuclei of water with deuterium.

Pump-probe spectroscopy measures a sample's response to a probe pulse at a controlled delay after an exciting pump pulse. Varying delay follows dynamics; selecting frequency and wave polarization can distinguish different contributions. Spectroscopic measurement of hydration dynamics is an application to water surrounding ions.

The symmetric filter with weights $(-1,2,4,2,-1)/6$ has unit gain at zero frequency, but its largest gain is $7/6$ at signed frequencies $\pm \pi /3$. Applied $k$ times, it multiplies a spectral density by $|H{|}^{2k}$. Thus repetition amplifies and selects a period-six oscillation instead of merely suppressing short-scale noise. On a two-sided frequency domain both peaks must be retained.

For a graph $z=x+i\nabla f$ in ${\mathbb{C}}^3$, symmetry of the Hessian matrix makes the graph Lagrangian. Pulling back $\Omega =d{z}^1\wedge d{z}^2\wedge d{z}^3$ gives $\det (I+i{D}^{2}f)$. Its imaginary part is $\mathrm{tr}{D}^{2}f-\det D^{2}f$, while its real part is $1-{\sigma }_2(D^{2}f)$. Vanishing of the imaginary part and positivity of the real part give the displayed equation and oriented branch. The equation is nonlinear even though its linearization near a flat graph is the Laplace equation.

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The bandwidth in this convention is the largest distance from the main diagonal of a nonzero matrix entry. A symmetric positive-definite matrix of bandwidth $b$ can be factored using $O(N{b}^2)$ operations and $O(Nb)$ storage; its Cholesky decomposition does not introduce entries outside that band. A variable permutation changes bandwidth, so it is a property of both the sparsity pattern and its ordering.

Compressed sparse storage keeps a sparse matrix as numerical nonzero values, their row or column indices, and pointers to the start of each compressed column or row. It avoids allocating all $N^2$ entries. A sparse direct solver must additionally allocate possible fill-in predicted by symbolic factorization, because elimination can create entries absent from the input.

If $H$ is formed by attaching one pendant edge to a fixed clique, each copy has a unique clique core. Its number of extensions is the number of ambient edges from that core to outside vertices. At a scale where the core count is tight and its expected extension count diverges sufficiently rapidly, a Chernoff bound uniformly over all possible cores shows that the total count, divided by the extension mean, has the same limiting law as the core count. Non-induced copies are intended; extra ambient edges are not forbidden.

Over a modular field or a complete discrete valuation coefficient ring, an indecomposable module has trivial source when a source is the rank-one trivial module of its vertex. These are precisely the indecomposable direct summands of permutation modules. A general trivial source module is a direct sum of these indecomposables.

Every finite-dimensional complex representation of a solvable Lie algebra has a common eigenvector. Equivalently, an irreducible finite-dimensional complex representation is one-dimensional; iterating gives simultaneous upper triangularization.

The solvable radical of a finite-dimensional Lie algebra is its largest solvable ideal of a Lie algebra. A Lie algebra is semisimple exactly when its solvable radical is zero.

The four-dimensional diamond Lie algebra is the semidirect product of Lie algebras of the three-dimensional Heisenberg Lie algebra with a derivation having eigenvalues zero, one and minus one. With basis $c,d,p,q$, its nonzero brackets are $[d,p]=p$, $[d,q]=-q$, $[p,q]=c$. Its derived series of a Lie algebra has dimensions four, three, one and zero. The symmetric form with $B(c,d)=B(p,q)=1$ and all other basis pairings zero is a nondegenerate bilinear form and invariant. The alternating tensor $B([x,y],z)$ is the volume form on $⟨d,p,q⟩$, verifying invariance.

When the Poisson equation for differential forms has one solution ${\alpha }_0$, all solutions are its translates by the kernel of a linear map of the Hodge Laplacian, namely the harmonic differential forms. The Hodge decomposition theorem identifies this translation vector space canonically with de Rham cohomology. There is no preferred affine origin until an extra normalization is chosen; orthogonality to harmonic forms picks the Green-operator solution.

An intergranular lane is a region between solar granules, commonly associated with cooler material and descending flow. Horizontal convergence can concentrate approximately vertical magnetic flux there. The field's geometry, diffusivity and feedback determine the actual concentration rather than the lane label alone.

The Babcock-Leighton mechanism regenerates large-scale poloidal solar flux through the emergence and dispersal of systematically tilted bipolar active regions. With Joy's law tilt, opposite polarities have different latitudes: cancellation near the equator and poleward transport of surviving following-polarity flux alter and reverse the polar field. Thus a toroidal field that produces rising bipolar regions feeds back into a poloidal magnetic field. The source is intrinsically nonlocal between the interior flux reservoir and surface emergence and is stochastic because active-region fluxes and tilts vary.

A flux-transport solar dynamo couples toroidal and poloidal field generation through meridional advection, turbulent magnetic diffusion and possibly turbulent pumping. A common model uses the surface Babcock-Leighton mechanism, differential rotation near the tachocline, poleward surface circulation and an equatorward deep return flow. If advection dominates, circulation strongly influences the cycle period and migration; if diffusion dominates, diffusive coupling is important instead. Agreement with a butterfly pattern depends on the assumed transport geometry and coefficients, not merely on including a circulation term.

An interface solar dynamo separates the region of strong differential rotation from the region of poloidal-field regeneration, often placing shear near the tachocline and an alpha effect in the convection zone. Magnetic transport across the interface couples the two regions. The separation can provide toroidal-flux storage in a relatively stable layer, but growth, period and migration depend on the transport, diffusivity contrast, and signs of the regeneration and shear coefficients.

Rotation influences convection, its helicity and differential rotation, which feed dynamo action. The generated magnetic field produces surface activity and strengthens the angular-momentum lever arm of a stellar wind. Wind braking lowers the rotation rate and ordinarily reduces unsaturated activity. Lorentz force feedback also changes the differential rotation and convective motions, saturating the dynamo on shorter time scales. Secular evolution, cyclic polarity reversal and irregular modulation are distinct time scales of this coupled system.

A solar butterfly diagram plots sunspot occurrence or magnetic flux against latitude and time. New-cycle sunspot belts appear at middle latitudes and migrate toward the equator, producing successive wing-shaped patterns. Magnetic butterfly diagrams additionally show poleward flux transport and polar-field reversals. The diagram constrains both Parker dynamo wave propagation and magnetic transport in flux-transport solar dynamos.

Bipolar solar active regions preferentially have one leading magnetic polarity in a given hemisphere and cycle, the opposite leading polarity in the other hemisphere, and reversed assignments in the next cycle. These statistical polarity rules reveal the ordered toroidal component of the solar magnetic field. Individual active regions can violate the preferred pattern, so the law is a statistical regularity rather than an exact rule for every spot pair.