A typed lambda term carries a type derivation under a context of typed free variables. The rules for variables, abstraction, and application correspond to assumptions, the implication introduction rule, and the implication elimination rule under the Implicational Curry-Howard correspondence. General fixed-point combinators belong to the untyped lambda calculus; self-application cannot be given a finite simple type.

Enumerate all computably enumerable sets as $W_e$. For each $e$, once a witness $x>2e$ appears in $W_e$, enumerate one such witness into $X$ and never act for $e$ again. Every infinite $W_e$ supplies a witness. Only requirements $e<n$ can put numbers into $[0,2n]$, so at least $n+1$ numbers in that interval stay outside $X$. Thus $X$ is a simple set. The proof uses a numerical bound rather than an infinitude test.

For a faithful primitive group action, suppose a point stabilizer has an abelian normal subgroup whose conjugates generate the whole group. Every nontrivial normal subgroup then contains the derived subgroup. Indeed the normal subgroup is transitive, and all those conjugates have the same image in the quotient, making the quotient abelian. If the whole group is nontrivial and perfect, it is simple.

A stationary mass density disturbance can be balanced by a changed azimuthal velocity in a shearing sheet. The continuity and azimuthal equations give zero radial velocity, while the radial equation balances Coriolis force against pressure and gravity. This mode is separate from the density-wave pair in the softened Toomre dispersion relation and is lost if one divides every equation by frequency.

For a steady point-source shear-driven viscous gravity current far downstream, $h\ll y_N\ll x$ permits dropping the downstream gravity flux. With $w=h^2$, the equation is $w_x=D(w^2{)}_{yy}/(4A)$, a quadratic porous medium equation. Conserving $A\int wdy=Q$ yields $y_N=(3\mu \Delta \rho gQx/{\tau }^2{)}^{1/3}$ and $h^2=\tau (y_N^2-y^2)/(2\Delta \rho gx)$ inside the current. The central height decays as $x^{-1/6}$. Near the source, balancing shear and gravity gives an upstream reach of order $\sqrt{\mu \Delta \rho gQ}/\tau$, whose numerical coefficient is not determined by scaling.

For a steady line-source shear-driven viscous gravity current with downstream constant thickness, $h_d^2=2\mu Q_{2d}/\tau$. Zero upstream volume flux per unit width gives $h{h}_x=3\tau /(2\Delta \rho g)$, so $h^2=h_d^2+3\tau x/(\Delta \rho g)$ until it vanishes at $x=-x_N$. Unlike the point-source scaling estimate, this reduced line-source model fixes the exact coefficient $2/3$. The square-root nose has an unresolved steep edge.

The sections are locally constant functions into a discrete set $T$, with ordinary restriction. It is the sheafification of the constant presheaf of sets. Its value on an empty open set is a singleton, even if $T$ is empty. For a topological space, this construction is left adjoint to the global sections functor: a map from it to a sheaf is a family of global sections indexed by $T$.

The space of germs of a presheaf has basic opens given by the germs of one section over an open set. Projection to its base point is a local homeomorphism. Continuous sections of this projection give sheafification. For the constant presheaf of sets, the basic opens are $U\times \{t\}$, giving the ordinary product with a discrete fibre.

For inclusion of a point $i:\{x\}\to X$, this is the direct-image sheaf of a set. It is right adjoint to the stalk functor. It uses the singleton terminal set on opens omitting $x$, rather than the zero group used for abelian-group skyscraper sheaves.

For a work-conserving single-server queue with independent Poisson arrivals, finite buffer and independent identically distributed admitted service times of mean $\mu$, Poisson arrivals see time averages give admitted rate $\lambda (1-P_n)$. Little law for the server alone then gives the displayed busy fraction. The time parameter is the mean service duration, and need not be exponentially distributed.

In a bipartite graph, a self-avoiding walk ends in its starting class exactly when it has even length. If degrees are bounded by $\Delta$, deleting the last edge gives ${\sigma }_{2k+1}\le \Delta {\sigma }_{2k}$ and hence $Z_G^0(x)\le Z_G(x)\le (1+\Delta x)Z_G^0(x)$ for $x\ge 0$. Thus these generating functions have the same radius of convergence.

Apply the oriented smoothing to every crossing of an oriented knot diagram, fill the resulting Seifert circles by disjoint disks at suitable heights, and reconnect them by twisted bands at the original crossings. The boundary is the original knot. If the connected surface has $s$ disks and $c$ bands, its Euler characteristic is $s-c$ and its genus is $(1-s+c)/2$.

A Seifert hypersurface for an oriented codimension-two submanifold is an oriented codimension-one submanifold having it as boundary. For a closed oriented surface in $S^4$, an integral meridional map from the complement to the circle, with angular behavior near the surface, supplies a Seifert hypersurface by taking a regular value.

A Hopf band is an annulus with one full twist whose boundary is a Hopf link. Plumbing it with another band adds a generator to the first homology of a Seifert surface. Its core has self-linking number $1$ or $-1$, depending on the twisting convention.

At a freely moving regular junction of three smooth image edges with equal isotropic length weights, the endpoint first variation requires the sum of their outward unit tangent vectors to vanish. Three unit vectors with zero sum have pairwise angles of $120$ degrees. Different interface weights instead give a weighted force balance, and constrained or singular junctions require separate hypotheses.

Relative tangential motion across a fracture in sea ice can break and pile blocks along the fracture. Local convergence at rough contacts makes ridging possible even when the large-scale deformation is mainly shear.

Here $n(H)dH$ is the expected number of ridge peaks per track length with sea-ice draft in $[H,H+dH]$. If total line density is $\mu$ and mean peak draft is $h_m>h_0$, integration gives $b=1/(h_m-h_0)$ and $B=\mu b{e}^{b{h}_0}$. The normalized peak draft has a shifted exponential distribution. This is a model over a specified draft range, not a universal description of every ridge.

A sonar ridge-identification rule can miss a shallow peak near a deeper peak because the intervening valley fails the required separation criterion. This apparent shadowing suppresses short observed spacings even without an acoustic blind zone. Finite beam width and genuine geometric occlusion can impose additional limitations. The resulting minimum resolvable spacing need not be a universal physical exclusion distance.

For a regression function $g_{\theta }$, let $h_{\theta }={\partial }_{\theta }{g}_{\theta }$. With independent centered normal distribution error of known variance ${\sigma }^2$, the parametric score function is $h_{\theta }(X)\epsilon /{\sigma }^2$. It has zero conditional expectation given the covariate, so unknown covariate-density nuisance directions do not remove any of its Fisher information.

Radial SLE grows from a boundary point to an interior target. In the unit disc from $1$ to $0$, its conformal-radius parameterization satisfiesThe target remains in the remaining simply connected component; the time parameter records its decreasing conformal radius.