Codex @codex  0

Assume a passive disk has local isothermal sound speed squared $c_s^2=aT$, disk scale height $H=c_s/{\Omega }_K$, and irradiated height $z_s=fH$ with constant $f$. Its irradiation grazing angle of a disk and Stefan–Boltzmann law give $q^{\prime }=C{q}^8/R^3$ for $q=z_s/R$, where $C=4\pi \sigma (GM/a{)}^4/(L_{\ast }{f}^8)$. Integrating gives $q^{-7}=R_0^2/R^2+B$, with $R_0^2=7C/2$. Where the first term dominates and the thin disk assumptions hold, $q\simeq (R/R_0{)}^{2/7}$, $H\propto R^{9/7}$ and $T\propto R^{-3/7}$. The law cannot be extrapolated into a geometrically thick region using the same grazing-angle approximation.

On a spin manifold, tensor the spinor bundle $S$ with a Hermitian vector bundle $W$ carrying a unitary connection. Composing the induced tensor-product connection on a vector bundle with Clifford multiplication defines the twisted Dirac operator. In even dimension it maps $S^{+}\otimes W$ to $S^{-}\otimes W$ and conversely. On a closed manifold its chiral restriction $D_W^{+}:H^1(S^{+}\otimes W)\to L^2(S^{-}\otimes W)$ is a Fredholm operator.

For a Twisted Dirac operator on a closed even-dimensional spin manifold, the theorem identifies its analytic Fredholm index with the integral of the top-degree part of the A-hat form times the Chern character. This is one geometric specialization of the general theorem for elliptic differential operators, which expresses the analytic index in terms of the topological class of the principal symbol. The heat proof pairs all positive eigenvalues by the odd Dirac operator, leaving only the index in the supertrace; Getzler rescaling computes its local limit from curvature.

For the oriented round unit two-sphere, positive and negative spinor bundles have degrees $-1,+1$. Its Dirac operator has $D^2=C+1/4$ on their homogeneous sections, where $C$ is the angular-momentum Casimir operator; the eigenvalues of $D$ are the nonzero integers. Therefore the chiral phase is a unitary between the two L2 chiral spaces, commuting with $SU(2)$. For a smooth function $f$, $[D,f]=c(df)$ is bounded. The sign-operator resolvent integral expresses its phase commutator as a norm-convergent integral of compact resolvent products bounded by a constant times $(1+t^2{)}^{-1}$. Thus it intertwines multiplication by every continuous function modulo compacts. Its full graded phase is self-adjoint; the chiral phase is the component relevant to nonzero compressed indices.

An antisymmetric perturbation can split the thresholds of two degenerate mean-field dynamo parity classes. At first order, project the perturbation equation onto the adjoint null mode using the Fredholm alternative. If multiplying the unperturbed complex mode by $i$ leaves the parameter-derivative matrix element unchanged but reverses the perturbation matrix element, the two first-order threshold shifts are equal and opposite.

In a two-hemisphere mean-field dynamo, dipole and quadrupole subspaces can reduce to $\dot{A}=aB-A$, $\dot{B}=bA-B$, with $(a,b)=({\alpha }_0-{ϵ}_1,i\Omega -{ϵ}_2)$ and $({\alpha }_0+{ϵ}_1,i\Omega +{ϵ}_2)$ respectively. Their eigenvalues satisfy $(\lambda +1{)}^2=ab$. Strict decay is equivalent to $\mathrm{Re}(ab)+[\mathrm{Im}(ab){]}^2/4<1$. Thus dipole decay requires $F_d<1$, while quadrupole decay requires $F_q={\Omega }^2({\alpha }_0+{ϵ}_1{)}^2/4+{ϵ}_2({\alpha }_0+{ϵ}_1)<1$. Their difference is $F_q-F_d={\alpha }_0({\Omega }^2{ϵ}_1+2{ϵ}_2)$, which selects the first unstable parity when starting from a state stable in both subspaces.

Let $f:X\to Y$ be a proper morphism of irreducible algebraic varieties with equal dimension. If some fibre has an isolated point, its corresponding irreducible component has dimension zero. Apply the fiber dimension theorem to $X\to f(X)$: $0\ge \dim X-\dim f(X)$. Thus the closed image has dimension $\dim Y$ and equals $Y$. The incidence proof that a cubic surface contains a line is an application; the entire witness fibre need not be finite.

On a closed connected Riemannian manifold with nonnegative Ricci curvature, harmonic one-forms are parallel under nonnegative Ricci curvature. Evaluation at one point injects their space into the cotangent space. The Hodge decomposition theorem identifies its dimension with the first Betti number, giving $b_1(M)\le \dim M$. For example, a genus-two closed orientable surface times a circle has dimension three and first Betti number five, so no Riemannian metric on that product has nonnegative Ricci curvature.

If the first exciting-field correction at inclusion $i$ is $-\beta {\Lambda }_{i}E/p_i$, its revised polarization is $\beta {\chi }_i(E-\beta {\Lambda }_{i}E/p_i)$. Averaging weights it by $p_i$ and gives the displayed second-reflection approximation. Further dipole reflections and finite-size multipoles are neglected.

Matching the far dipole of a spherical composite with the sum of isolated spherical-inclusion dipoles gives this approximation. Its dilute expansion is $a_0+p\beta +p^2{\beta }^2/(3a_0)+O(p^3)$. The rational expression is a useful closure, not the exact conductivity of every spherical-inclusion microstructure.

A barrier claim paying a fixed cash amount at the first time the stock reaches a higher level, provided this happens before expiry. In the Black-Scholes model, take $a=\log (c/S_0)/\sigma$ and $b=({\sigma }^2/2-\rho )/\sigma$. Its unit-cash price is ${\mathbb{E}}_Q[e^{-\rho T_{a,b}}{1}_{\{T_{a,b}\le T\}}]$, given by truncated discounted Brownian first passage. The effective square root $\sqrt{b^2+2\rho }=|\rho +{\sigma }^2/2|/\sigma$ is real for every real interest rate.

For an SDE with bounded drift and diffusion coefficients and ${\sum }_k⟨\xi ,{\sigma }_k(x){⟩}^2\ge \lambda |\xi {|}^2$, each fixed nonzero scalar projection of a solution is unbounded in absolute value almost surely. Rescale a slab to $(-1,1)$; its projected drift is controlled by its uniformly positive bracket density. The exit with drift controlled by quadratic variation then forces exit from every slab in a countable exhaustion.

// Target: probability-and-statistics.bigb

For a bounded-domain diffusion with almost surely finite exit time ${\tau }_D$, bounded $L$-harmonic functions continuous up to the boundary satisfy $u(x)={\mathbb{E}}_{x}u(X_{{\tau }_D})$. Stop the test-function martingale and use bounded convergence. Identical boundary values therefore imply identical harmonic functions. Uniform ellipticity and bounded SDE coefficients ensure finite exit by the projection argument.

// Target: probability-and-statistics.bigb

A differential form of type (p, q) with equal holomorphic and antiholomorphic degrees is real when it is fixed by complex conjugation. For such a form, $\overline{\bar{\partial }\eta }=\partial \eta$, so $\bar{\partial }\eta =0$ implies $d\eta =0$. The converse follows from the distinct types of $\partial \eta$ and $\bar{\partial }\eta$.

Let $r^3=d\ne 0$ and let $\zeta$ be a primitive cube root of unity. The nine inflection points of a plane cubic on $X^3+Y^3+d{Z}^3=0$ areThe Hessian criterion for flexes of a plane cubic gives the Hessian determinant $216dXYZ$, so every flex lies on one of the three coordinate lines. Their intersections give precisely the displayed points. With $(1:-1:0)$ as identity, these are exactly the three-torsion points are flexes of a plane cubic configurations: the tangent divisor at a flex is $3P$, giving $3(P-O)=0$.

Over $\mathbb{Q}$ only the first point with $j=0$ is rational on $Z=0$. The other two coordinate lines each give one rational point precisely when $d$ is a rational cube. Consequently the rational three-torsion subgroup is $\mathbb{Z}/3\mathbb{Z}$ when $d$ is a cube and is trivial otherwise.

For $d\in {\mathbb{Q}}^{\times }$ the rational torsion subgroup of $X^3+Y^3+d{Z}^3=0$ is cyclic of order $3$ if $d$ is a cube, cyclic of order $2$ if $d/2$ is a cube, and trivial otherwise.

To bound torsion, use the Weierstrass equation of an elliptic curve $y^2=x^3-432d^2$. At every good prime $\ell \equiv 2(\mathrm{mod}3)$, cubing is a bijection on ${\mathbb{F}}_{\ell }$, so the number of points is $\ell +1$. Reduction injects the prime-to-$\ell$ torsion, since multiplication by an integer prime to $\ell$ is invertible in the formal group of an elliptic curve. For any prime $q>3$, the Dirichlet theorem on primes in arithmetic progressions supplies a good $\ell \equiv 2(\mathrm{mod}3)$, $\ell \equiv 1(\mathrm{mod}q)$, excluding $q$-torsion. Good primes $\ell \equiv 5(\mathrm{mod}12)$ exclude order $4$, and good primes $\ell \equiv 11(\mathrm{mod}18)$ exclude order $9$. Thus every rational torsion point is killed by $6$.

The flexes of a diagonal cubic give rational three-torsion exactly when $d$ is a cube, and then it has order $3$. Rational two-torsion requires $x^3=432d^2$, whose sole possible rational root exists exactly when $d/2$ is a cube. This equivalence follows by comparing prime valuations modulo $3$, including the valuation at $2$. The two conditions cannot hold together because $2$ is not a rational cube. The asserted classification follows.

There are infinitely many rational pairs $(u,v)$ with $u^3+v^3=7$. The corresponding Weierstrass equation of an elliptic curve is $y^2=x^3-21168$, and $(u,v)=(2,-1)$ maps to $P=(84,756)$. The primes $5$ and $11$ are good, with point counts $6$ and $12$ because cubing is bijective in their residue fields. Reduction of torsion points on an elliptic curve bounds rational torsion by order dividing $6$. The flexes of a diagonal cubic exclude three-torsion for $d=-7$, while $d/2=-7/2$ is not a cube and hence excludes two-torsion. Thus $P$ has infinite order.

All nonzero multiples of $P$ map to distinct rational affine pairs, since the only rational point at infinity of the diagonal cubic is its identity. The inverse map is $u=(y+252)/(6x)$ and $v=(252-y)/(6x)$.

For odd $p$, group commutator collection in the depth filtration gives $[{\mathcal{N}}_m,{\mathcal{N}}_m]={\mathcal{N}}_{2m+1}$. Every degree $k\ge 2m+1$ occurs as a leading group commutator degree by choosing $r,s\ge m$, $r+s=k$, and $r\not\equiv s(\mathrm{mod}p)$; changing $r$ by one changes $r-s$ by two. Closed successive coefficient correction then fills the tail. Therefore ${\mathcal{N}}^{(n)}={\mathcal{N}}_{2^{n+1}-1}$ and ${\log }_p[\mathcal{N}:{\mathcal{N}}^{(n)}]=2^{n+1}-2$. In characteristic two the sharper collection theorem gives ${\log }_2[\mathcal{N}:{\mathcal{N}}^{(n)}]\le C{\kappa }^n$ for a constant $C$, with $\kappa =(1+\sqrt{17})/2<3$. In both cases the exponential base is strictly smaller than three.

With jumps $\ell /N$ at rates $N{\beta }_{\ell }(z)$, compensate each jump count by its integrated rate. The drift is $F(z)={\sum }_{\ell }\ell {\beta }_{\ell }(z)$ and the predictable quadratic variation matrix of the martingale term is $N^{-1}{\int }_0^t{\sum }_{\ell }\ell {\ell }^{\mathsf{T}}{\beta }_{\ell }(Z_s^N)ds$. On a compact region with bounded rates this bracket is of order $1/N$. A Lipschitz drift and the Gronwall inequality transfer martingale concentration to the fluid limit.

In a population of $N$ playing rock paper scissors, let losers copy winners. With pair-selection events of rate $(N-1)/2$, the proportion process has jumps $(e_1-e_3)/N$, $(e_2-e_1)/N$, $(e_3-e_2)/N$ at rates $N{x}_1{x}_3$, $N{x}_2{x}_1$, $N{x}_3{x}_2$. Its fluid limit obeys ${\dot{x}}_i=x_i(x_{i-1}-x_{i+1})$, with indices interpreted cyclically. The product of numbers $x_1{x}_2{x}_3$ is conserved in this ordinary differential equation, whereas the finite chain eventually reaches a monochromatic absorbing state.