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A nonterminal is unreachable when no word derivable from the start symbol contains it. Productions whose left sides are unreachable do not affect the generated language.

A discrete valuation ring is a principal ideal domain with exactly one nonzero maximal ideal. Every nonzero element of its fraction field is a unit times a unique integer power of a uniformizer.

Let a finite group $G$ of order $d$ act analytically on a compact Riemann surface, and let an invariant analytic map $F$ have degree $d$. At a point with stabilizer of order $e$, invariance forces the local multiplicity of $F$ to be at least $e$. Its orbit has $d/e$ points, so it already contributes at least $d$ to the fibre multiplicity. The valency theorem forces equality and shows that every fibre is exactly one orbit.

The rotation group of the octahedron has order $24$. Its vertex, face-centre, and edge-centre stabilizers have orders $4,3,2$, giving exceptional orbit sizes $6,8,12$; every other orbit has size $24$.

The degree of a nonconstant elliptic function is the sum of the orders of its poles in a fundamental parallelogram. Equivalently, it is the degree of the induced holomorphic map from its complex torus to the Riemann sphere.

After cancelling common factors, the rational map $p/q$ on the Riemann sphere has degree $\max (\mathrm{deg}p,\mathrm{deg}q)$. It is an analytic isomorphism exactly when this degree is one, equivalently when it is a Möbius transformation.

For a nonconstant holomorphic map $f:X\to Y$ between compact connected Riemann surfaces, its degree isThe valency theorem says that this integer is independent of $y\in Y$.

If a nonconstant holomorphic map has local-coordinate expressionthen $m=m_f(p)$ is its local degree, multiplicity, or ramification index at $p$.

This is the symmetric-particle construction of Fock space.

For one-particle Hilbert space ${\mathcal{H}}_1$, the bosonic Fock space isCreation operators add one symmetrized particle and annihilation operators remove one.

Every continuous real-valued function on a closed subspace of a normal space extends continuously to the whole space. If its values lie in a closed interval, the extension can be kept in that interval.

If three nonproportional linear forms exclude three distinct residue classes modulo every sufficiently large prime, the upper-bound sieve has dimension three and gives an upper bound of order $x/(\log x{)}^3$.

When an off-block subdiagonal entry tends to zero, QR iteration asymptotically separates the corresponding invariant spectral blocks even if eigenvalues within one block have equal modulus.

The accumulated orthogonal factor in QR iteration is the $Q$ factor of a matrix power, so its leading columns span the same spaces as simultaneous power iteration.

An unshifted QR step preserves the bandwidth of a real symmetric banded matrix. A Givens-rotation implementation exposes this as bulges created during triangularization and removed when the factors are multiplied in reverse order.

LetThenThus ${\overline{Q}}_k{\overline{R}}_k$ is a QR decomposition of the matrix power $A^{k+1}$. In particular, the first $r$ columns of ${\overline{Q}}_k$ span the same subspace as the first $r$ columns of $A^{k+1}$ whenever those columns are independent.

For a finite Galois extension of local fields, the lower ramification groups measure how closely automorphisms fix the valuation ring. The inertia group is $G_0$, and $G_1$ is the wild inertia group.

Let $X$ have contractible universal cover, let $K$ be a path-connected simplicial complex, and let $i:K^1\hookrightarrow K$. A map $f:|K^1|\to X$ extends to $|K|$ exactly when its induced fundamental-group map factors through $i_{\ast }$. The factorization kills every 2-simplex boundary; all higher-dimensional boundary maps lift to the contractible universal cover and are null-homotopic.

For a path-connected locally path-connected base, any two based universal covers lift to one another. The two composites and the respective identity maps are based lifts of the same covering maps, so uniqueness of lifts makes the composites identities. The lift is therefore a homeomorphism.

Rational homology satisfiesIt records the free rank of integral homology and discards torsion.

The determinant $U(n)\to S^1$ sends a unitary loop to a circle-valued loop. A nonzero determinant winding number proves that the original loop class has infinite order.