A Kakeya tube is a neighborhood of transverse radius $\delta$ of a line segment of fixed unit length. Round cross-sections and comparable rectangular cross-sections are interchangeable up to fixed constants. The tube has Lebesgue measure comparable to ${\delta }^{n-1}$ in ${\mathbb{R}}^n$.

The dimension assertion asks that every bounded Kakeya set in ${\mathbb{R}}^n$ have full Minkowski dimension. The Kakeya maximal conjecture implies the stronger lower-dimension conclusion: its application to $E_{\delta }$ gives $|E_{\delta }|{\gtrsim }_{n,\epsilon }{\delta }^{n\epsilon }$, hence $N_{\delta }(E){\gtrsim }_{n,\epsilon }{\delta }^{-n+n\epsilon }$ for every $\epsilon >0$.

The number of joints of a line collection formed by $L$ distinct real affine lines in ${\mathbb{R}}^n$ is at most $C_n{L}^{n/(n-1)}$. The pruning and minimal-degree polynomial argument proves this bound. The exponent is sharp: the coordinate-line grid has $m^n$ joints of a line collection and $n{m}^{n-1}$ lines. In three dimensions the bound is $C{L}^{3/2}$.

Every continuous $2\pi$-periodic function admits degree-at-most-$n$ trigonometric polynomials with supremum norm error bounded by a universal constant times its modulus of continuity at scale $1/n$, for $n\ge 1$. This first-order Jackson-type estimate transfers to algebraic polynomials through cosine substitution for polynomial approximation.

A Zp-extension is an infinite Galois extension of a number field whose Galois group is topologically isomorphic to the additive p-adic integers. There is a unique intermediate field of degree $p^n$ for every $n\ge 0$, corresponding to $p^n{\mathbb{Z}}_p$. The finite layers form a tower with cyclic successive degree $p$ extensions.

An Iwasawa module is a module over an Iwasawa algebra. In arithmetic towers it usually comes from a norm inverse limit of units or class groups, or from a compact Galois module for a maximal abelian pro-$p$ extension. Specify which extension is allowed to ramify: the p-ramified Iwasawa module and the unramified Iwasawa module have different ranks.

A map on $(0,1)$ can satisfy $0<f(x)<x$ and still have no orbit tending to zero. Partition the domain into $I_j=(1/(j+1),1/j]\cap (0,1)$ and define $f(x)=(x+1/(j+1))/2$ on $I_j$. Every interval is invariant under the iteration of a map, and $f^n(x)=1/(j+1)+2^{-n}(x-1/(j+1))$ has a positive limit. The discontinuities at interval boundaries prevent that limit from being a fixed point. In contrast, continuity of a self-map satisfying $f(x)<x$ permits passage to a positive orbit limit and would force an impossible fixed point.

A nonempty closed subset which, with its induced topology, is an irreducible topological space. In an affine algebraic set these subsets correspond, with reversed inclusion, to prime ideals of the coordinate ring.

The value of an investment portfolio, obtained by summing each holding times its current asset price. Consumption removes money from portfolio wealth; a self-financing portfolio changes its value only through asset gains. Nonnegative portfolio wealth is a common admissibility restriction in an investment-consumption problem.

The first Jackson theorem for periodic approximation applied to $|\sin \theta |$ gives algebraic best error $O(n^{-1})$ for $\sqrt{1-x^2}$. Its ordinary interval modulus of continuity at $1/n$ is at least $\sqrt{2/n-1/n^2}$. Substituting that error rate into the unmodified inverse theorem for trigonometric approximation would instead bound the modulus by $O(\log (n+1)/n)$, a contradiction. Algebraic inverse estimates must incorporate the endpoint compression of cosine substitution for polynomial approximation.

A smooth gradient flow confined to a compact invariant sublevel set has finite total integral of $\Vert \nabla V{\Vert }^2$ when $V$ is bounded below. Compactness supplies uniform continuity of that squared norm along the trajectory, so it tends to zero. Every accumulation point is a critical point. If the trapped component contains exactly one critical point, the trajectory converges to it. This supplies a direct convergence proof in place of an inference solely from an energy-contour picture.

For closed irreducible subvarieties $Y,Z$ of a smooth $d$-dimensional algebraic variety $X$, every irreducible component of their intersection has dimension at least $\dim Y+\dim Z-d$. Locally the diagonal of $X\times X$ is cut out by $d$ parameters. Intersecting it with $Y\times Z$ and applying the Krull height theorem gives the bound. The three-dimensional affine quadric cone shows that the same lower bound can fail when $X$ is singular.

For a plane inertia-gravity wave independent of one horizontal coordinate, period-mean kinetic and available potential energy obey $⟨K⟩-⟨A⟩=2⟨K_y⟩$. The transverse rotating velocity is in quadrature with the in-plane velocity. The modified oscillator balance is $⟨K_{xz}⟩=⟨A⟩+⟨K_y⟩$; ordinary kinetic/potential equipartition occurs only when the transverse amplitude vanishes.

For $\mathcal{L}={\dot{\varphi }}^2/2-{\mathcal{V}}_2+{\mathcal{L}}_3(\varphi ,\dot{\varphi })$, where ${\mathcal{L}}_3$ is cubic in perturbation amplitude, the canonical momentum has $\pi =\dot{\varphi }+O({\varphi }^2)$. Put $\dot{\varphi }=\pi +\Delta$ with $\Delta$ second order. Then $\pi \dot{\varphi }-{\dot{\varphi }}^2/2={\pi }^2/2-{\Delta }^2/2$, so through cubic order the Legendre transform in mechanics gives ${\mathcal{H}}_3=-{\mathcal{L}}_3$ evaluated with the free momentum. At fourth order there are extra terms, so derivative interactions do not obey this simple rule at all orders.

A heavy real scalar $S$ with quadratic mass $M$ and interaction $-aS{\phi }^2/2$ can be removed at tree level using $(M^2+\square )S=-a{\phi }^2/2$. Substituting into both its source and quadratic terms gives the displayed interaction. Its derivative expansion starts with $a^2{\phi }^4/(8M^2)-a^2{\phi }^2\square {\phi }^2/(8M^4)$. For a light quartic written as $-\lambda {\phi }^4/4!$, the matched quartic is $\lambda -3a^2/M^2$. The expansion is valid for momentum invariants small compared with $M^2$.

An element $x$ of an $A$-algebra is an integral element precisely when $A[x]$ is a finite $A$-module. More generally, a finite $A$-submodule containing one and stable under multiplication by $x$ proves integrality by the determinant trick. Conversely a monic polynomial equation reduces all powers of $x$ to finitely many generators.

For a compact connected regular level of $n$ independent Poisson-commuting functions, the periods of their joint flow form a full-rank discrete subgroup $\Lambda \subset {\mathbb{R}}^n$. The level is ${\mathbb{R}}^n/\Lambda$. A local smooth basis of these periods determines action integrals and the normalized periodic fields used to construct action-angle variables.

On the regular set of two uncoupled oscillators with irrational frequency ratio, every continuous globally defined integral of motion is constant on each fixed-action flat torus, because every orbit is dense there. A differentiable integral is consequently a function of the two actions, so its differential of a smooth map lies in their span. Local angle combinations can still give extra local integrals; they fail to be single-valued on the torus.

For a periodic inextensible filament with arclength period $\Lambda$ and projected axial period $\lambda$, material points slide tangentially through the wave frame at speed $Q$. An axial pattern speed $V$ relative to mean filament motion gives $Q\lambda /\Lambda =V$. Confusing the two speeds changes the swimming-to-wave-speed ratio by $\lambda /\Lambda$.

In the compact picture of a principal-series group representation of the Lorentz spinor double cover, functions on SU(2) have a fixed right-torus character $e^{-in\vartheta }$. Expanding into spin matrix coefficients leaves one right weight in each allowed spin, giving the displayed multiplicity-one rotation decomposition. The boost action couples these spaces. The central element acts by $(-1{)}^n$; even $n$ yields integer rotation spins and a group representation of the connected Lorentz group.