nLab perfectoid space

Contents

Contents

Idea

Perfectoid spaces are a variant of Huber spaces in analytic geometry. The concept was introduced (Scholze 11) in order to generalize the classical theorem of (Fontaine-Winterberger 79) (see also at function field analogy).

This theorem establishes an isomorphism between the absolute Galois groups of the extension L n p[p 1/p n]L \coloneqq \cup_{n}\mathbb{Q}_{p}[p^{1/p^{n}}] of the p-adic numbers and of the perfection L n𝔽 p((t 1/p n))L^{\flat} \coloneqq \cup_{n}\mathbb{F}_{p}((t^{1/p^{n}})) of the field of Laurent series of the finite field 𝔽 p\mathbb{F}_p (the isomorphism between Galois groups continues to hold after passing to the completions of these fields - it is these completions that are more properly called perfectoid). The field L L^{\flat} is called the tilt of LL. In (Scholze 11) this is generalized by the statement that the perfectoid spaces over fields related this way are equivalent. (See Bhatt 14 for a review).

Definitions

The reference for these definitions are Weinstein17.

Perfectoid fields

  • A nonarchimedean field KK of residue characteristic pp is perfectoid if its value group is nondiscrete and the pp-th power Frobenius map on K /pK^{\circ}/p is surjective (here K K^{\circ} is the subset of power bounded elements).

Perfectoid rings

  • A Huber ring is a topological ring AA which contains an open subring A 0A_{0} which is adic with respect to a finitely generated ideal of definition IA 0I\subseteq A_{0}.

  • A Huber ring AA is Tate if it contains a topologically nilpotent unit ϖ\varpi (also called a pseudouniformizer).

  • A Tate Huber ring is perfectoid if it is complete, uniform (the subset of power-bounded elements is bounded), and contains a pseudo-uniformizer ϖ\varpi such that ϖ p|p\varpi^{p}\vert p and the p-th power map A/ϖA/ϖ pA/\varpi\to A/\varpi^{p} is an isomorphism.

Perfectoid space

  • A perfectoid space is an adic space which can be covered by affinoids Spa(A,A +)Spa(A,A^{+}), where AA is a perfectoid ring.

Properties

Tilting

The concept of tilting allows us to generalize the theorem of Fontaine and Wintenberger on the isomorphism of the absolute Galois groups of a perfectoid field and its tilt mentioned above, to perfectoid rings, and further to perfectoid spaces.

Definition

(ScholzeWeinstein20, Definition 6.2.1)

Let RR be a perfectoid ring. The tilt of RR is

R =limxx pRR^{\flat}=\underset {\underset{x\mapsto x^{p}}{\leftarrow}} {\lim}R

A priori this is a topological multiplicative monoid, so turn it into a topological ring we equip it with the addition structure given by

(x+y) (i)=lim n(x (i+n)+y (i+n)) p n.(x+y)^{(i)}=\lim_{n\to\infty}(x^{(i+n)}+y^{(i+n)})^{p^{n}}.

Proposition

(ScholzeWeinstein20, Theorem 7.4.5)

Let RR be a perfectoid ring and let R R^{\flat} be its tilt.

  • For any finite etale RR-algebra SS, SS is perfectoid.

  • Tilting SS S\mapsto S^{\flat} induces an equivalence between finite etale RR-algebras and finite etale R R^{\flat}-algebras.

  • For any finite etale RR-algebra SS, the algebra S S^{\circ}, is almost finite etale over R R^{\circ}.

The tilting construction “glues” and therefore carries over from perfectoid rings to perfectoid spaces; in other words to a perfectoid space XX we can take the tilt to obtain a perfectoid space X X^{\flat} of characteristic pp (ScholzeWeinstein20, 7.1).

Proposition

(ScholzeWeinstein20, Corollary 7.5.3)

There exists an etale site X etX_{et} such that X etX et X_{et}\cong X_{et}^{\flat} and H i(X et,𝒪 X +)H^{i}(X_{et},\mathcal{O}_{X}^{+}) is almost zero for all i1i\geq 1 and for all affinoids XX.

Generalizations

The concept of perfectoid space can be generalized into that of a diamond, which is a quotient of a perfectoid space of characteristic pp by a perfectoid equivalence relation (ScholzeWeinstein20, Definition 8.3.1). The concept of diamond can similarly be generalized into that of a v-sheaf, and in particular a small v-sheaf is a quotient of a diamond by a diamond equivalence relation (ScholzeWeinstein20, Proposition 17.2.2).

Applications

One application of perfectoid spaces is in relating Galois representations to torsion in the cohomology of Shimura varieties. In turn, using the excision sequence, one can relate this to the cohomology of arithmetic manifolds that are not Shimura varieties (for example Bianchi manifolds, which are quotients of hyperbolic 3-space by an arithmetic subgroup). This is surveyed in section 5 of Weinstein15.

References

Exposition includes

  • Michael Harris, The perfectoid concept: Test case for an absent theory (pdf)

The concept is due to

motivated by

Review includes

Perfectoid spaces and related concepts were the topic of a course at Berkeley in 2014, whose lecture notes have now been made into a book:

Some applications of perfectoid spaces are discussed in

  • Jared Weinstein, Reciprocity Laws and Galois Representations: Recent Breakthroughs pdf

See also

Formalization in type theory (in Lean):

Last revised on November 29, 2022 at 19:02:26. See the history of this page for a list of all contributions to it.