For a finite cell decomposition of the sphere into polygonal faces, the numbers of vertices, edges and faces satisfy the displayed formula. Triangulating faces preserves $V-E+F$ because each added diagonal adds one edge and one face. This is the spherical form of Euler formula for a connected planar graph, where the plane's exterior face is included.

In the ordered basis $(J,E_1,E_2)$ of the Euclidean motion Lie algebra in two dimensions, the Killing form has matrix $\mathrm{diag}(-2,0,0)$. Its radical is the translation ideal, and the algebra is solvable rather than semisimple.

For a bounded self-adjoint operator $L$ on a complex Hilbert space, defineThis equals the spectrum of a bounded operator after removing its isolated eigenvalues of finite multiplicity. To see isolation when the kernel is finite and the shifted range is closed, restrict the shifted operator to its kernel complement: it is invertible there by image-kernel orthogonality for an adjoint. A Neumann-series perturbation leaves nearby nonzero shifts invertible on that complement and on the finite kernel. The closed range must concern the shifted operator, not the original one.

For an intersecting family of $r$-sets with $n\ge 2r$, the rank-$r$ lower shadow of its complement family is disjoint from the original family. Iterating the Kruskal-Katona theorem shows that exceeding $\left(\genfrac{}{}{0ex}{}{n-1}{r-1}\right)$ original members would force this shadow to have more than $\left(\genfrac{}{}{0ex}{}{n-1}{r}\right)$ members, contradicting the total number $\left(\genfrac{}{}{0ex}{}{n}{r}\right)$ of $r$-sets.

Positive integers are equivalent when they have the same finite set of prime factors. Equivalently, each divides a positive power of the other. This equivalence relation is induced by the radical of an integer; its equivalence classes correspond to finite prime supports. The empty support gives the singleton $\{1\}$, and every nonempty support gives an infinite class by varying exponents.

If finite nonnegative entropy $H(t)$ is locally absolutely continuous and decreases, its limit $H_{\infty }$ exists. The fundamental theorem of calculus applied to $H^2$ proves the displayed absolute-integrability identity. This product conclusion does not require a zero limiting entropy.

The inequality $H\le I/2$ and the entropy dissipation identity for Ornstein-Uhlenbeck flow imply $H^{\prime }\le -2H$. An integrating factor proves the displayed estimate from initial entropy alone. Pinsker's inequality then gives $\Vert f_t-\gamma {\Vert }_1\le \sqrt{2H(f_0\mid \gamma )}{e}^{-t}$.

For a dilute nonrelativistic gas with $x=m/T\gg 1$ and zero chemical potential, the Bose-Einstein distribution approaches the Maxwell-Boltzmann distribution. Expanding $E=m+p^2/(2m)+\cdots$ in the number density integral gives $n\sim g(mT/(2\pi ){)}^{3/2}{e}^{-x}$. Since $\rho =mn+3nT/2+\cdots$ and $P=nT$, the entropy density at zero chemical potential gives $s=n(x+5/2+\cdots )$ and hence the displayed leading expression. This includes the internal multiplicity $g$; a single real scalar field has $g=1$.

The displayed asymptotic uses natural units $k_B=c=\hslash =1$. Thus $m$ and $T$ are both energy scales, and $x=m/T$ is dimensionless. Restoring SI units replaces this ratio by $m{c}^2/(k_{B}T)$ and includes $k_B$ in the physical entropy per particle.

A comonad is an endofunctor $G$ with natural transformations $ϵ:G\to 1$ and $\delta :G\to G^2$ satisfying $Gϵ\delta ={ϵ}_G\delta =1_G$ and $G\delta \delta ={\delta }_G\delta$. It is the categorical dual of a monad. Its coalgebras for a comonad carry compatible maps into their image under $G$.

For smooth ideal magnetohydrodynamics of constant mass density, define $I_{\pm }={\int }_V|{\mathbf{z}}^{\pm }{|}^{2}dV$. Taking the dot product of the Elsässer variable equation with $2{\mathbf{z}}^{\pm }$ givesTherefore each integral is constant when its outward boundary flux vanishes, for example when both velocity and magnetic field are tangent to the fixed boundary, or for periodic boundary conditions. The kinetic energy plus magnetic energy is $\rho (I_{+}+I_{-})/4$, while the cross-helicity is $\sqrt{{\mu }_0\rho }(I_{+}-I_{-})/4$.

In vacuum-permeability magnetohydrodynamics, the magnetic energy in a volume is $E_B=\int B^2/(2{\mu }_0)dV$. Its volume density equals the magnetic pressure.

Bragg scattering is coherent elastic scattering from a periodic lattice, enhanced when the wavevector transfer is a reciprocal lattice vector. Finite crystals have narrow rather than infinitely sharp reciprocal-space peaks.

A surface Ekman layer is the stress-driven boundary correction below a rotating fluid’s upper boundary. For upward $z\le 0$, $f>0$, constant kinematic viscosity and imposed complex wind stress $T$, its steady velocity is $W_E=T{e}^{\lambda z}/(\rho \nu \lambda )$, $\lambda =(1+i)/{\delta }_E$. The underlying geostrophic flow is independent of $T$ unless an additional boundary velocity condition couples them.

For $f>0$, a stationary lower no-slip boundary condition below uniform exterior geostrophic flow $W_g$ produces $W_E=-W_g{e}^{-(1+i)z/{\delta }_E}$. The upward shear component at the bottom is $T_b=\rho \nu W_E^{\prime }(0)=\rho \sqrt{f\nu /2}(1+i)W_g$; the bottom traction on the fluid is $-T_b$. This law cannot be assigned to an arbitrary prescribed surface wind stress without an additional boundary condition.

For abelian $G$ and $n\ge 1$, the identity of $G$ determines this class under $H^n(K(G,n);G)\cong \mathrm{Hom}(G,G)$. Pulling it back realizes representability of cohomology by Eilenberg–MacLane spaces. A map inducing this class on a fiber induces the identity of its unique positive homotopy group.

For a CW complex $X$, abelian $G$ and $n\ge 1$, homotopy classes of maps to the Eilenberg–MacLane space correspond to cohomology classes by pullback of the universal cohomology class of an Eilenberg–MacLane space. Pointed maps give the reduced version. This converts a cohomology class into an actual map realizing its effect on homotopy.

The ring is exterior on its degree-$n$ universal class for odd $n$, and polynomial for even $n$. The path-loop Serre spectral sequence alternates these two types by transgression. In the polynomial-fiber case its derivation has nonzero coefficients $k$ on powers, which become invertible over the rationals.

For $n\ge 1$, the mod-two cohomology of $K(\mathbb{Z}/2,n)$ is polynomial on the admissible operations in the displayed strict-excess range, including the empty operation. It is obtained from path-space transgressions and compatibility with Steenrod squares. Products must still be included when listing low-degree vector-space bases.

For $n\ge 4$, the first four possible degrees have integral groups $\mathbb{Z}/2,0,\mathbb{Z}/2,\mathbb{Z}/2$ in degrees $n,n+1,n+2,n+3$. Mod-two Betti numbers count cyclic summands, while the nonzero first Bockstein actions on $u,{\mathrm{Sq}}^{2}u,{\mathrm{Sq}}^2{\mathrm{Sq}}^{1}u$ show that their orders are two. This prevents mistaking a nonzero mod-two group for a higher-order integral cyclic group.

The spin-one-half octet of flavor symmetry contains the nucleons, Sigma baryons, Lambda baryon, and Xi baryons. Its weight diagram has six peripheral states and two states at $(I_3,Y)=(0,0)$: ${\Sigma }^0$ and ${\Lambda }^0$, with different total isospin.