Let $Y$ be the reduced union of the irreducible components of an affine variety $X$ containing $P$. Removing all other components leaves an open neighbourhood of $P$ on which $X$ and $Y$ agree, so their local rings at $P$ agree. In the coordinate ring $B=k[Y]$, every denominator not vanishing at $P$ avoids all minimal primes: each component of $Y$ contains $P$. Such a denominator is a non-zero-divisor. Consequently $B_{{\mathfrak{m}}_P}$ injects into the total quotient ring $Q(B)=\mathrm{Rat}(Y)$. One cannot generally replace $Y$ by a single component through an intersection point.

A global unit and its inverse are algebraic integers in the number field. Equivalently its valuation is zero at every nonarchimedean prime. Global units embed in local unit groups; their p-adic closures can be smaller than the full local groups. This discrepancy is measured by the class-field unit sequence.

Let ${\partial }_{\pm }={\partial }_{\tau }\pm {\partial }_{\sigma }$ and $\mathcal{L}={\partial }_{+}X\cdot {\partial }_{-}X+i{\psi }_1\cdot {\partial }_{+}{\psi }_1+i{\psi }_2\cdot {\partial }_{-}{\psi }_2$. For constant Grassmann-odd parameters, set $\delta X=i({ϵ}_2{\psi }_1-{ϵ}_1{\psi }_2)$, $\delta {\psi }_1=-{ϵ}_2{\partial }_{-}X$, $\delta {\psi }_2={ϵ}_1{\partial }_{+}X$. In the first chiral sector the variation of the fermionic term cancels $i{ϵ}_2{\partial }_{+}{\psi }_1\cdot {\partial }_{-}X$ from the bosonic term, leaving $i{ϵ}_2{\partial }_{-}({\psi }_1\cdot {\partial }_{+}X)$. The second sector gives the other total derivative. This proves off-shell invariance when the boundary flux vanishes. Closure on fermions uses their free equations of motion. A constant parameter must also preserve the chosen boundary conditions and spin structure.

The hypotenuse is the side opposite the right angle in a right triangle. Its length $c$ is determined by the Pythagorean theorem.

The classical right ring of quotients inverts all regular elements of a ring. Existence requires the right Ore condition for these denominators. The original ring embeds, every regular element becomes a unit, and every fraction has a right denominator. The right Ore condition also permits finitely many fractions to be placed over a common right denominator.

A multiplicative set $S$ containing $1$ and not $0$ is a right denominator set if it satisfies the right Ore condition and right reversibility: $sr=0$, with $s\in S$, implies $rt=0$ for some $t\in S$. Its Ore localization consists of right fractions $r{s}^{-1}$ and has kernel $\{r:rt=0\text{ for some }t\in S\}$. This condition permits zero-divisor denominators; it is automatic for a set of regular elements of a ring. When all denominators are regular, the localization map is injective.

A right-handed neutrino is a singlet of the entire Standard Model gauge group. Its same-chirality bilinear with the field obtained by charge conjugation therefore admits the displayed renormalizable Majorana mass term, in addition to a Higgs-induced Dirac mass term. It violates lepton number by two units. Analogous terms for electrically charged Standard Model fermions carry nonzero electric charge; quarks also have nontrivial color charge. The presence of a right-handed field permits mass but does not force a nonzero mass parameter.

A semiprime ring has a classical right ring of quotients that is a semisimple ring and Artinian exactly when it is a right Goldie ring. In particular, the theorem applies to a semiprime ring that is a right Noetherian ring. It extends the fraction-field construction from commutative domains to rings whose quotient can be a finite product of matrix rings over division rings.

In a positively oriented Riemannian orthonormal frame write $X={\sum }_i{f}_i{E}_i$, with dual coframe ${\omega }_1,\dots ,{\omega }_n$. Define the omitted-factor differential formsThe Riemannian volume form is ${\omega }_g={\omega }_1\wedge \cdots \wedge {\omega }_n$, and its interior product isInserting $E_i$ into the first argument moves it past $i-1$ factors, producing the sign. For $n=2$, this gives ${\iota }_X({\omega }_1\wedge {\omega }_2)=f_1{\omega }_2-f_2{\omega }_1$.

For positive coordinate charts put $J=\partial y/\partial x$. The metric tensor transforms by $g_x=J^T{g}_{y}J$, giving $\det g_x=(\det J{)}^2\det g_y$. The top wedge product of differential forms transforms by $d{y}^1\wedge \cdots \wedge d{y}^n=(\det J)d{x}^1\wedge \cdots \wedge d{x}^n$. Since $\det J>0$, the two factors match. Thus the local expressions define one global Riemannian volume form.

The total Riemannian volume is the integral of the Riemannian volume form, or its density on a nonorientable manifold. In local coordinates the density is $\sqrt{\det g}d{x}^1\cdots d{x}^d$. It is preserved by Riemannian isometries. On a closed manifold it is the leading coefficient among the integrated heat invariants, and on a surface it is the surface area.

For a surjective Riemannian submersion $\pi :M\to N$, let $H={\sum }_{\alpha }({\nabla }_{V_{\alpha }}{V}_{\alpha }{)}^{\mathrm{hor}}$ be the unnormalised mean curvature vector of its fibres, using an orthonormal vertical frame. With the positive Laplace-Beltrami operator, tracing the Riemannian Hessian in horizontal and vertical directions gives ${\Delta }_M(f\circ \pi )=({\Delta }_{N}f)\circ \pi +d(f\circ \pi )(H)$. Horizontal traces agree with those on the base; a vertical Hessian term is $-d(f\circ \pi )({\nabla }_{V_{\alpha }}{V}_{\alpha })$. Thus minimal fibres, in particular totally geodesic submanifolds as fibres, make pullback an injection from each base eigenspace into the corresponding total-space eigenspace. Its image is exactly the eigenfunctions constant on each whole fibre.

At any point of a Riemannian manifold, choose geodesic normal coordinates and orthonormalize their coordinate fields smoothly. At the point, the metric is the identity and its first derivatives vanish, so the orthonormalization coefficients have zero first derivatives. The Christoffel symbols also vanish there. This gives a local Riemannian orthonormal frame with the displayed property, useful for invariant identities evaluated at one point.

A compact manifold with boundary is simple when the boundary is strictly convex and any two points are joined by a unique geodesic depending smoothly on the endpoints, without conjugate points. Geodesics exit in finite time. These properties make off-diagonal boundary distance derivatives and the geodesic scattering relation well behaved.

The map $(x,r)\mapsto {\exp }_x(r{\nu }_{\mathrm{in}})$ gives a collar in which the metric has this form. Two metrics with the same induced boundary metric can be identified by their normal collars, fixing the boundary and matching full tangent-space metrics there. To extend the local identification globally, interpolate the metrics, form their smoothly varying collars, and extend the resulting collar isotopy velocity by a cutoff. The time-dependent vector field vanishes on the boundary and agrees with that velocity near it, so its global flow gives the desired boundary-fixing diffeomorphism.

A Riemannian manifold is locally symmetric when its Riemann curvature tensor is parallel for the Levi-Civita connection. Along a geodesic, parallel transport then conjugates the Jacobi curvature operator at the initial point to the operator at every later point. Thus its eigenvalues stay constant. The condition is local and does not assert that global point reflections extend to isometries of the whole manifold.

For a connected complete Riemannian manifold of dimension $n\ge 2$ and constant sectional curvature $k>0$, its complete simply connected universal cover is the round sphere of radius $k^{-1/2}$. This is the constant-curvature case of Cartan-Ambrose-Hicks theorem. Thus the manifold is a quotient by a finite group of isometries acting freely. In even dimension, the determinant embeds this group in $\{1,-1\}$: every determinant-one orthogonal transformation in odd ambient dimension has eigenvalue one and therefore a fixed point. The only possible nonidentity element is the antipodal involution, giving Real projective space. In odd dimensions, free cyclic scalar actions on complex unit spheres instead produce lens spaces.

A linear isometry between tangent spaces of complete simply connected Riemannian manifolds extends to a global isometry when parallel transport along corresponding broken geodesics identifies their Riemann curvature tensors. In particular, complete simply connected manifolds of the same dimension and constant sectional curvature are isometric: their curvature tensors are determined by that common constant and their Riemannian metrics. This is the constant-curvature form often called Cartan's theorem.

The chordal metric on the Riemann sphere is $\chi (z,w)=|z-w|/\sqrt{(1+|z{|}^2)(1+|w{|}^2)}$ for finite points, with $\chi (z,\infty )=(1+|z{|}^2{)}^{-1/2}$. It is half the Euclidean chord distance between stereographic images on the unit sphere, so the triangle inequality follows from the Euclidean one. If $\rho$ is round spherical geodesic distance, then $\rho =2\arcsin \chi$ and $2\chi \le \rho \le \pi \chi$. These inequalities transfer uniform-continuity estimates between spherical and chordal distances, including at poles of meromorphic functions.

For a smooth compact complex surface, the degree-four part of its Todd class is $(c_1^2+c_2)/12$. Applying Riemann–Roch to its structure sheaf gives the formula. For a K3 surface, $c_1=0$ and $\chi ({\mathcal{O}}_X)=2$, so its topological Euler characteristic is $c_2=24$.