Milnor K-theory and birational anabelian reconstruction

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Milnor K-theory and birational anabelian reconstruction

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Take two vector spaces over F_ℓ, V = K₁ᴹ/ℓ and W = K₂ᴹ/ℓ, together with their product. This class shows how that small amount of data rebuilds a whole function field of transcendence degree at least two. It also explains why the rebuilt field is fixed only up to one scalar in F_ℓ^× and Frobenius.

The statement [0:45]

Let K/k and L/l have algebraically closed constants, with characteristic ≠ ℓ and trdeg ≥ 2. A linear isomorphism Θ: V_K → V_L is compatible when (Θ⊗Θ)(R_K) = R_L, which means it respects the Steinberg relations. Compatible maps up to scalars then correspond bijectively to isomorphisms of perfect closures up to Frobenius. Neither ambiguity can be removed. Scaling Θ by a squares the degree-two map. On V, Frobenius acts as multiplication by p. At ℓ = 2 the scalar is trivial.

Three steps that carry the proof [1:50]

Alternation gives valuations. A pair of characters f, g is alternating when f(x)g(1−x) = f(1−x)g(x). This condition is dual to the Steinberg relations, so Θ preserves it. The largest alternating subspaces have dimension trdeg, and two of them meet in the inertia line of a quasi-divisorial valuation.
Bounding what mod ℓ forgets [3:01]. In [t^ℓ(t−1)] = [t−1], the zero of order ℓ disappears. On a residual curve of bounded genus, Riemann–Hurwitz over five anchor fibers gives (5(1−1/ℓ)−2)n′ ≤ 2G_X − 2 + ΣN_i. At ℓ = 2 the left coefficient is ½ with five fibers and 0 with four, so five is the count that works for every prime [4:08].
Incidence descent builds curves [4:39]. The supporting divisors of Θ([t−a]) have bounded degree, so they lie in one finite-type family. Restrict the incidence variety to a curve T, pass to the compositum L·k(T), then descend. The result is Θ(V_E) = V_P.

Next, true divisors and rational subfields recover the field operations, which produces α: K^i → L^i with Θ = c·α_* [5:27].

A move to reuse: find the relation the map preserves, bound what reduction mod ℓ forgets, then let families of divisors build the curves [5:48].

Research preprints

Milne's rationality conjecture for abelian varieties

BSD leading-term formula from low Selmer corank

Selmer converse and sums of two cubes

The two-primary BSD formula in rank one

Zero-free regions and the quasi-Riemann hypothesis

The quasi-Riemann hypothesis and zero-free regions

Landau–Siegel zeros: how a determinant could forbid a real zero near 1

Converse theorem for elliptic curves with 2-torsion

Hilbert's tenth problem over the rationals

Why Catalan's constant G is irrational

Goldfeld's conjecture and the 2∞-Selmer converse

Average rank of elliptic curve twists

Chowla–Elliott correlations via divisibility graphs

The Grothendieck–Teichmüller Lie algebra and the Deligne–Drinfeld conjecture

Milnor K-theory and field reconstruction mod p

Milnor K-theory and birational anabelian reconstruction

Bogomolov–Pop reconstruction: the uniform bound along a pencil

Fontaine–Mazur modularity at the prime 2

The dimension of the two-adic Hecke algebra

Pro-modularity at 2 without de Rham conditions

Beating the parity obstruction for primes p with μ(p − 1) = 1

Poisson-Dirichlet law for prime factors of p−1

Totient function fibers and smooth shifted primes

Independence of largest prime factors in consecutive integers

Ostmann's inverse Goldbach problem and prime indecomposability

Hecke eigensheaves with tame ramification

Frobenius structures on tame Hecke eigensheaves

Global Arthur parameters via cuspidal excursion data

Nilpotent orbit decomposition of cuspidal automorphic functions

Rationality of the canonical Arthur filtration

Hecke eigensheaves with ramification at marked points

Temperedness at ramified places for exceptional groups

Restricted geometric Langlands in positive characteristic

Equidistribution of prime-degree torus packets

Sextic torus packets: why degree six needs a new machine

Equidistribution of quartic torus packets as orders grow

Zilber–Pink for curves in A2: elliptic squares

Finiteness of quaternionic points on Siegel curves

Why E × CM points run out: the exponent race behind Zilber–Pink for curves in A2

The abelian Zilber–Pink conjecture: atypical intersections and height scales

Irrationality exponent of π and Flint–Hills convergence

Normal subgroups and the Margulis–Platonov conjecture

The Margulis–Platonov conjecture over number fields

Section conjecture for p-adic curves

Étale covers with a prescribed exterior sheet

Power-free values of irreducible quartic to octic polynomials

Jacobsthal's function and the quadratic bound

The weak inhomogeneous Duffin–Schaeffer problem

Kummer's bias in cubic Gauss sums

Counting totient values: Ford's asymptotic refined

Short Egyptian fraction expansions and Erdős's bound

Positive density of large prime gaps

Zariski density of integral points on character varieties

Gaussian moats: why bounded-step prime walks must stop

Artin's primitive root conjecture explained

Simultaneous primitive roots: conditional bound

Modularity of elliptic curves over imaginary quadratic fields

Uchida's conjecture: from local correspondences to Hoshi's cyclotomic test

A conditional reduction for algebraic Kuga–Satake correspondences

Hodge conjecture for self-powers via Gale duality

Hodge conjecture for K3 self-powers via Kuga–Satake

Hodge classes on CM abelian varieties

Hodge conjecture on abelian variety powers

Kähler canonical bundle formula and b-semiampleness

Logarithmic Kodaira dimension and whole-fiber variation

Iitaka subadditivity and Campana's orbifold conjecture

Iitaka's subadditivity via Hodge-theoretic techniques

Logarithmic Kodaira additivity and the reverse inequality

Abundance after nonvanishing for compact Kähler fourfolds

Bounded Stein degree for log Calabi–Yau boundaries

Good minimal models for Kähler fourfolds

Canonical nonvanishing for fourfolds via moving jets

Lifting sections for log abundance in fourfolds

Log abundance for compact Kähler spaces

Log abundance for log canonical pairs in characteristic zero

Minimal singular metrics on nef adjoint bundles

Uniform Cartier bounds in canonical bundle formulas

Pseudo-effectivity and Kodaira dimension of fiber spaces

Effective Iitaka fibrations and uniform pluricanonical degree