The stationary squared process has the positive series $X_t^2={\alpha }_0{\sum }_{k\ge 0}{\alpha }_2^k{\prod }_{j=0}^k{\epsilon }_{t-2j}^2$. Even and odd observations therefore use disjoint families of independent random variables. This explains zero lag-one covariance of the squares despite dependence at lag two.

The doubly infinite ladder graph has graph vertices $\mathbb{Z}\times \{0,1\}$, horizontal edges $\{(j,s),(j+1,s)\}$, and vertical edges $\{(j,0),(j,1)\}$.

For $0<c\le w_k<\infty$, interpret the nonnegative sum as an extended-valued functional on the l2 sequence space. Finiteness implies $u\in {\ell }^1$, but unbounded weights can make it infinite even there. It is proper and convex. Every finite partial sum is norm-continuous, so their supremum is sequentially lower semicontinuous and also weakly lower semicontinuous. This is useful as a sparsity penalty in variational regularization.

For spaces whose integral cohomology has only free and $\mathbb{Z}/2$ summands, let $F$ encode free ranks and $T$ count order-two summands by degree. An additively split integral Künneth theorem gives $F_{A\times B}=F_A{F}_B$ and the displayed rule for $T$. The $t^{-1}$ term is the cohomological degree shift of the Tor functor contribution. The formula requires this torsion type and the usual finite-type hypotheses.

The singular-chain product induces this natural pairing on homology. Composing it with multiplication of a loop space gives the Pontryagin product on loop-space homology. This is different from the vector cross product.

For $d\ge 1$ and $n>1$, no factor-permutation-invariant map $f:(S^d{)}^n\to S^d$ restricts to the identity on the diagonal. The Künneth theorem makes the degree-$d$ cohomology a direct sum of $n$ copies of $\mathbb{Z}$. Symmetry forces $f^{\ast }\alpha =a{\sum }_i{u}_i$ for one integer $a$. Pulling back to the diagonal multiplies this coefficient by $n$, so the identity restriction would give $na=1$. The contradiction uses integral coefficients and applies in both even and odd sphere dimensions.

If only the columns $p=-1,0$ are nonzero, a cohomological differential of bidegree $(r,1-r)$ has no possible nonzero source-target pair for $r\ge 2$. Projective boundaries in a complex of projectives give this condition because every cohomology module has projective dimension at most one.

A finite-dimensional module is ${⨁}_a{M}_a^{\oplus m_a}$ with pairwise nonisomorphic indecomposables and uniquely determined multiplicities. The Fitting lemma makes their endomorphism rings local. The multiplicity spaces provide the general linear factors in the Levi decomposition of a quiver automorphism group.

The Krivine rounding scheme yields $c_K=0.56109985\dots$ as its guaranteed objective factor. It is obtained by normalizing $\sinh t=1$, so $t=\mathrm{arsinh}(1)$ and $c_K=2t/\pi$. This proof gives a valid universal factor for the bipartite sign problem, not a proof that it is optimal.

For bipartite binary quadratic optimization, if $v^{\ast }$ is the sign optimum and $p_{\mathrm{SDP}}^{\ast }$ its semidefinite relaxation of binary quadratic optimization value, then $c_K{p}_{\mathrm{SDP}}^{\ast }\le v^{\ast }\le p_{\mathrm{SDP}}^{\ast }$. The upper bound follows from rank-one lifting. For the lower bound, Krivine rounding scheme preprocessing followed by Gaussian hyperplane rounding has expected objective exactly $c_K{p}_{\mathrm{SDP}}^{\ast }$, and every rounded vector is feasible. Some outcome reaches at least this expectation. No independence of rounded coordinates or positive semidefiniteness of the original bipartite objective is needed.

With zero cosmological constant and the component Hodge star operator convention $(\star j{)}_{abc}={ϵ}_{abcd}{j}^d$, the Einstein field equations and Killing equation give $d\star dm=-16\pi \star (T_{ab}{m}^b-\frac{1}{2}T{m}_a)$. If $m$ is tangent to a spacelike hypersurface, the trace term pulls back to zero. For $\partial \Sigma =S_{\infty }-{\bigcup }_i{S}_i$, Stokes theorem yieldsThe inner Komar charges cannot be dropped for a vacuum Kerr black hole, whose angular momentum is nonzero although its stress-energy tensor vanishes. A matter-only volume formula requires a regular filling with no inner boundaries.

A one-sided section loop and a two-sided fiber loop in the reflection mapping torus have mod-two self-intersections one and zero and meet once. Their intersection form is $Q$. In the evaluation-dual cohomology basis, the Poincare duality pairing has matrix $Q^{-1}$, giving $t^2=0$ and $x^2=tx\ne 0$.

For a transverse complex velocity amplitude $\hat{\mathbf{u}}$ and real nonzero wavevector $\mathbf{k}$, write $\hat{\mathbf{u}}=\mathbf{a}+i\mathbf{c}$. Solenoidality makes both real vectors perpendicular to $\mathbf{k}$, so $\frac{1}{2}\mathrm{Re}(i{\hat{\mathbf{u}}}^{\ast }\times \hat{\mathbf{u}})=-\mathbf{a}\times \mathbf{c}=H\mathbf{k}$. The corresponding spatial average of kinetic helicity density is $-H{k}^2$. Cross-product order determines this sign.

A force-free, torque-free microswimmer in a stationary Newtonian fluid, with negligible fluid and body inertia and steady Stokes flow at each instant, gains no net displacement from a reciprocal stroke: one whose sequence of shapes is exactly retraced in reverse. The environment and boundary conditions must also be unchanged during that retracing. Opening a two-shell swimmer and then closing it follows the same sequence backwards; doing the closure faster does not help, because Linearity of Stokes flow changes the instantaneous velocity by the same factor as the stroke rate.

A single real shape coordinate that travels out and back along an interval gives this cancellation, as computed in the force-free two-sphere stroke. Two independently controlled shape coordinates can instead execute a loop that is not retraced, avoiding this obstruction. Continuous rotation also need not retrace an interval: the helical microswimmer with a spherical head provides another route to propulsion. The scallop theorem is a restriction on a stroke under stated conditions, not a claim that small organisms cannot swim. Purcell presented the opening-and-closing scallop and a swimmer with two hinges in Life at low Reynolds number (1977), figures 6–7.

For a smooth passing encounter of a force-driven sphere with a fixed sphere, isotropy and reflection symmetry imply $\mathbf{U}=[m_{\perp }(R)I+(m_{\parallel }(R)-m_{\perp }(R))\mathbf{nn}]\mathbf{F}$. Along a force axis $X$, $U_X$ is even and the transverse velocity is odd in $X$. Uniqueness then makes the trajectory fore-aft symmetric and restores the incoming impact parameter downstream. This consequence of kinematic reversibility of Stokes flow remains valid with lubrication resistance, provided the particles remain smooth, noncontacting and subject only to the stated forces. Surface roughness and contact can invalidate these assumptions.

Outside a subextremal Kerr black hole, regular causal curves obey $(-g_{t\varphi }-\sqrt{D})/g_{\varphi \varphi }\le d\varphi /dt\le (-g_{t\varphi }+\sqrt{D})/g_{\varphi \varphi }$, where $D=g_{t\varphi }^2-g_{tt}{g}_{\varphi \varphi }>0$. Positive radial and polar metric terms establish this necessary bound. In the open Kerr ergoregion both endpoints are positive for $a>0$, forcing co-rotation.

Superradiance is amplification of a scattered wave by extracting rotational or other available conserved energy from a background. For a rotating Kerr black hole, a bosonic mode of frequency $\omega$ and azimuthal integer $m$ can have negative horizon Killing-energy flux when $0<\omega <m{\Omega }_H$. The reflected energy can then exceed the incident energy. A stationary Killing flow need not define a globally positive particle energy.

For bound old Kepler orbits, $E<0$, $\lambda <0$ and $\bar{E}>0$ give a positive radius, positive isochrone scale $b$, and positive transformed mass. The shifted energy is $\bar{E}-k=\bar{E}/\lambda <0$. For unbound old orbits with $E>0$, $1/2<\lambda \le 1$ gives an attractive unbound isochrone branch. The case $\lambda =0$ is handled by the separate Kepler–harmonic radial duality.

Two deep layers of incompressible flow with upper mass density ${\rho }_1$, lower mass density ${\rho }_2$, and tangential velocities $U_1,U_2$ have this dispersion relation when surface tension is absent. Decaying velocity potentials and the linearized kinematic boundary condition determine their amplitudes; the dynamic boundary condition for an inviscid interface gives the displayed relation. For $k>0$ the squared departure of the phase velocity $c$ from the density-weighted mean velocity is $g({\rho }_2-{\rho }_1)/[k({\rho }_1+{\rho }_2)]-{\rho }_1{\rho }_2(U_1-U_2{)}^2/({\rho }_1+{\rho }_2{)}^2$. A negative value gives Kelvin-Helmholtz instability despite stable density stratification.

The intrinsic planetary luminosity $L_{\mathrm{int}}$ is the power supplied by interior cooling, contraction, nuclear reactions if present, or internal heating, excluding reflected or passively reradiated stellar light. For fixed mass and no accretion or nuclear source, global conservation of energy gives $L_{\mathrm{int}}=Q_{\mathrm{deep}}-d(U+\Omega )/dt$, where $Q_{\mathrm{deep}}$ is any additional power deposited in the interior, such as tidal heating or Joule heating. For $Q_{\mathrm{deep}}=0$ and a gas-supported quasi-static sphere of fixed specific-heat ratio $\gamma$ and negligible surface pressure, the stellar virial theorem gives $\Omega =-3(\gamma -1)U$, henceFor a dynamically stable gas-supported model take $\gamma >4/3$. For $\gamma =5/3$ this is half the gravitational release. Degenerate giants require a different internal energy reservoir; the half-factor is not universal.