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The number $2\cos (\pi /n)$ is an algebraic integer. If it is rational it must be an integer, which shows that $\cos (\pi /n)$ is rational only for $n=1,2,3$.

The Chebyshev polynomial of the second kind is defined byIt is orthogonal on $[-1,1]$ with weight $\sqrt{1-x^2}$ and has roots $\cos (j\pi /(n+1))$ for $1\le j\le n$.

For a constant uniform magnetic field $B$, choose the symmetric gauge $A=\frac{1}{2}B\times r$. The coordinate parallel to $B$ is cyclic, so its canonical momentum is conserved. Since $A\cdot B=0$, this component is also the mechanical momentum $m\dot{r}\cdot \hat{B}$.

Under $\varphi \mapsto \varphi -{\partial }_{t}f$ and $A\mapsto A+\nabla f$, the charged-particle Lagrangian changes byIt therefore has unchanged equations of motion by total-time-derivative invariance.

Independence makes the characteristic function of a sum equal the product of the individual characteristic functions.

Purely imaginary characteristic roots produce oscillatory solutions. In particular, $r\tau =-\pi /2$ gives $\lambda =\pm i\pi /(2\tau )$ and the real solution $\cos (\pi t/(2\tau ))$.

The degree of an irreducible character $\chi$ is $\chi (1)$, the dimension of the corresponding irreducible representation.

For a discrete memoryless channel with $m$ output symbols, suppose every row of the transition matrix is a permutation of every other row and all column sums agree. Such a communication channel hasThe row Shannon entropy is the same for every input, so $H(Y\mid X)$ has that fixed value for every input probability distribution. The remaining term $H(Y)$ is at most ${\log }_{2}m$ by maximum entropy on a finite alphabet.

A uniform input attains the bound. If there are $\ell$ input symbols, the total of all transition probabilities in the matrix is $\ell$; equal column sums must therefore each be $\ell /m$. Multiplication by the uniform input mass $1/\ell$ gives output mass $1/m$ in each column. Hence $H(Y)={\log }_{2}m$, which proves the displayed formula. The cyclic ternary channel capacity, binary symmetric channel capacity, and ternary symmetric channel capacity are concrete applications of this symmetry calculation.

Center-of-mass reduction separates translational motion from internal coordinates by using the conserved or externally forced center coordinate.

If $G/Z(G)$ is cyclic, then $G$ is abelian. Indeed, if $gZ(G)$ generates the quotient, every element is $g^{a}z$ with $z\in Z(G)$, and any two such elements commute.

The word metric measures how many allowed moves are needed to change one group element into another. For a finite symmetric generating set of a group $S$ with $1\notin S$, it is the graph distance between vertices of the Cayley graph, equivalentlyHere word length counts letters in a shortest representative of $g^{-1}h$. The path metric on the entire Cayley graph also permits points inside edges; the word metric is its restriction to the group itself.^[ref]

The word metric is left invariant:This corresponds to the label-preserving group action on the Cayley graph. Symmetry follows by reversing a shortest graph path; concatenating routes gives the triangle inequality.^[ref]

Right invariance can fail in a non-abelian group, because right multiplication replaces $g^{-1}h$ by $a^{-1}{g}^{-1}ha$ through conjugation. In the free group $F(x,y)$ with $S=\{x,x^{-1},y,y^{-1}\}$, we have $d_S(1,y)=1$, whereas $d_S(x,yx)=|x^{-1}yx{|}_S=3$: the latter freely reduced word has three letters. In an abelian group, conjugation fixes each element, so right invariance does hold. Cayley graphs of the integers and word metric balls in the integer lattice give examples where the word metric can be calculated explicitly.

For an order-$k$ scalar quasilinear partial differential equation with real-analytic coefficients, real-analytic Cauchy data on a real-analytic non-characteristic hypersurface determine a unique real-analytic solution in a neighbourhood of each point of that hypersurface.

For a scalar equation of order $k$, Cauchy data consist of the first $k$ normal jets $u,{\partial }_{N}u,\dots ,{\partial }_N^{k-1}u$ on the initial hypersurface.

These prescribed values define the initial conditions of a Cauchy problem.

For $0<s<a$,

For $a>0$ and real $c$, integrate $e^{-a{z}^2}$ around a wide rectangle between the real axis and $\mathrm{Im}z=c$. The vertical contributions vanish as the width tends to infinity, so

If $f$ is analytic and bounded by $K$ on $\mathrm{Re}z>0$, then Cauchy's derivative estimate giveson $\mathrm{Re}z>c$. Integrating along line segments gives a Lipschitz bound with the same constant in that smaller half-plane.

A functor $F:\mathcal{C}\to \mathcal{D}$ is a discrete fibration when every arrow $B\to FA$ has a unique lift with codomain $A$.

A functor $F:\mathcal{C}\to \mathcal{D}$ is final when every comma category $(B\downarrow F)$ is nonempty and connected. Colimits are unchanged after restriction along a final functor.

The category $\mathbf{Met}$ has metric spaces as objects and maps $f$ satisfying $d(fx,fy)\le d(x,y)$ as morphisms. Its binary product uses the maximum metric.

For $F:\mathcal{C}\to \mathcal{D}$ and $B\in \mathcal{D}$, the objects of $(B\downarrow F)$ are arrows $B\to FA$; its morphisms are commuting triangles induced by arrows in $\mathcal{C}$.