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Construction of Hodge Theaters

Yu Yang

RIMS, Kyoto University

September 2, 2021

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In this talk, I will explain the first paper of Prof. Mochizuki’s theory of IUT. The talk is divided into three parts as follows:

The motivation of Hodge theaters The goal of Hodge theaters

The construction of Hodge theaters (in particular, the ´etale picture!)

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§1: Motivation of Hodge theaters

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F/Q: a number field

OF: the ring of integers ofF

E/F: an elliptic curve s.t. E admits a semi-stable model E/OF

V(F): the set of places of F

V(F)non,bad: the set of non-arch. placesv s.t. Ev def

= E|Fv has bad reduction, where Fv denotes the local field at v

ℓ: a prime number distinct frompv for all v∈V(F)non,bad, wherepv denotes the characteristic of the residue field of Fv

htE: Faltings height of E

Goal: We hope that htE can be bounded forallelliptic curves satisfying the above conditions.

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Note thatEv =Gm/qZv ↞Gm ⊇µ for all v∈V(F)non,bad, and that 0→µ →Ev[ℓ]Z/ℓZ1.

Global multiplicative subspaces (=GMS)

We shall callH ⊆E[ℓ] a “GMS” ifH|Fv coincides withµ for all v∈V(F)non,bad. This means that there exists a Galois ´etale covering Y →E corresponding to H such that Yv →Ev is a topological covering of dual semi-graphs for all v∈V(F)non,bad.

If “GMS” exists, then by some standard discussions of Diophantine geometry, we may obtain that htE can be bounded. However, we have

#{E/F s.t. GMS exists}<∞.

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Goal of IUT: We want to do similar discussions forarbitrary elliptic curves over number fields.

The first step: We need an analogue of “GMS” forarbitrary elliptic curves.

IUT’s answer:It’s “Hodge theaters”

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§2: Goal of Hodge theaters Reference

Section 1 of “S. Mochizuki, The ´etale theta function and its

Frobenioid-theoretic manifestations. Publ. Res. Inst. Math. Sci. 45 (2009), 227-349.”

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Local theory (over non-archimedean bad places) ℓ >>0: a prime number

p≥3: a prime number s.t. p̸=ℓ k: ap-adic field

Ok: the ring of integers ofk

X∼=Gm,k/qZ: an elliptic curve overkwith bad reduction (i.e., Tate curve)

Xlog: the log stable curve overk determined by the zero point ofX Moreover, we assume that

√−1∈k

X[2ℓ](k) =X[2ℓ](k), where kdenotes an algebraic closure ofk

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Then we have the following commutative diagram of cartesian squares of tempered coverings:

Y¨log −−−−→µ Y¨log

µ2



y µ2y Ylog −−−−→µ Ylog

Z



y Z

 y

Xlog −−−−→µ Xlog −−−−→Z/ℓZ Xlog

The above coverings are defined in the next page via a picture of special fibers.

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The special fibers of the above commutative digram is as follows:

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On the other hand, let us fix a cusp OX (i.e., zero cusp) ofX. Then the image of OX ofX →X is the zero cusp (or the zero point)OX. The curve(X, OX) can be regarded as an elliptic curve overk. Thus, we obtain

Xlog −−−−→Z/ℓZ Xlog

±1



y ±1y Clog −−−−−→degree Clog, where Clog def= [

Xlog/{±1}]

andClog def= [

Xlog/{±1}]

denote the quotient stacks.

Moreover, there exists a unique irreducible component 0Xs Irr(Xs) such that the reduction of OX is contained in0Xs, where Irr(Xs) denotes the set of irreducible components ofX.

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Let 0Ys Irr(Ys) be a lifting of0Xs. Then we obtain a labeling Z Irr(Ys) Irr( ¨Ys) Irr( ¨Y

s) such that 07→0Ys. Moreover, we put

µ∈X(k): 2-torsion point whose reduction is contained in0Xs µY∈Y(k): the unique lifting of µ s.t. the reduction is contained in 0Ys

ξjY ∈Y(k): j·µY (with the action ofj Z=Aut(Ylog/Xlog))

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We have the following definition.

Definition 1

We shall call a lifting of ξjY ∈Y(k) in Y¨(k)an evaluation point ofY¨log labeled byj Z( Irr(Ys)). Moreover, we shall call a lifting of an evaluation point of Y¨(k) labeledj in Y¨(k)an evaluation point ofY¨log labeled byj Z( Irr( ¨Ys) Irr(Ys)).

Moreover, we have the following picture:

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We have the following diagram of special fibers:

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Non-archimedean Θ-functions We put

Y¨, Y, Y¨, Y

thep-adic formal schemes whose Raynaud generic fibers areY¨,Y,Y¨,Y, and whose special fibers are Y¨s,Ys,Y¨s,Ys, respectively.

Write 0Y¨

s Irr( ¨Ys) for the irreducible component over0Ys Irr(Ys) and U ⊆¨ Y¨,U ⊆ Y for the open formal subschemes such that

U¨s= 0Y¨s\Y¨ssing (=Gm), Us = 0Ys \Yssing (=Gm).

Then U is isomorphic to the p-adic formal completion of Gm,Ok with multiplicative coordinate U Γ(U,OU). Moreover, we put

U¨ def=

U Γ( ¨U,OU¨).

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We have the following function on Y¨: Θ( ¨¨ U) =q18 ·

n∈Z

(−1)n·q12(n+12)2 ·U¨2n+1. Moreover, we define a function

Θdef= ¨Θ(an evaluation pt labeled by 0)·Θ¨1

onY¨ which can be regarded as an “ℓ-th root” ofΘ¨ (in the sense of cohomological classes). Note that there are exactly two evaluation points labeled by0 in Y¨s, and that we have

Θ(an ev. pt labeled by¨ 0) =Θ(another ev. pt labeled by¨ 0).

Values of Θat evaluation points

We putq def= q2ℓ1. Then Θ(an evaluation pt labeled byj)∈µ2ℓ·qj2.

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Let V(F)non,bad be the notation introduced in§1. Moreover, we denote by Θv the function defined above at the placev∈V(F)non,bad such thatv is not over2. Then we have

±1 ±2 . . . ±j . . . Θv 7→ q

v q4

v . . . qj2

v . . .

The Goal of Hodge theaters:Roughly speaking, Hodge theater (at least, the ´etale part) is a virtual “GMS” for an arbitrary elliptic curve over a number field which manages

Θv-values for all non-archimedean bad places (with their labels) via anabelian geometry.

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§3: Initial Θ-data Reference

Section 3 of “S. Mochizuki, Inter-universal Teichm¨uller theory I:

Construction of Hodge theaters. Publ. Res. Inst. Math. Sci. 57(2021), 3–207.”

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Firstly, we have the following notation:

F :a number field s.t.

1∈F

E :an elliptic curve overF s.t. E has stable reduction at all v∈V(F)non

ℓ>5 :a prime number s.t. ℓ̸=pv for all v∈V(F)non,bad K def= F(E[ℓ])

Fmod:the field of moduli ofE Xdef= E\ {OE}

C def= [X/1}]

Furthermore, we assume that

E[6](F) =E[6](F), whereF denotes an algebraic closure ofF CK def

= F K is a “K-core” (i.e., a terminal object in the category of ´etale coverings and quotients of XK overK)

F/Fmod is Galois

SL2(F)Im(GF Aut(E[ℓ]) (=GL2(F))

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Let XK →XK def

= F K be an ´etale covering with Galois group Z/ℓZ. Note that #(XcptK \XK) =ℓ. Just like the local theory recalled above, we fix a cuspOXK ∈XcptK \XK and call it zero cusp. Then(XcptK , OXK) is an elliptic curve over K. In particular, there exists a{±1}-action on XK. Thus, we have

XK −−−−→Z/ℓZ XK

±1



y ±1y CK −−−−−→degree CK,

where CK def= [XK/{±1}]denotes the quotient stack. Moreover, we fix a non-zero cusp

ϵ

of CK (i.e., the image of a cusp∈XcptK \(XK∪ {OXK})).

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We put the following

VV(K) : a subset s.t. the natural mapV,→V(K)↠V(Fmod) is a bijection

Vbad V:a non-empty subset s.t. for everyv Vbad, the following are satisfied: (i)E has bad reduction atv; (ii)Xv →Xv induces a topological covering of their dual semi-graphs; (iii) the reductionϵv of ϵis the cusp ofClogv labeled by 1F/{±1} (=Cusp(Clogv )); (iv) the image of the natural map Vbad,→V(K)↠V(Fmod)↠V(Q)does not contain 2. Then for eachv Vbad, we have the local theory explained in §2.

Vgood def= V\Vbad. Note that Ev,v∈Vgood, has bad reduction in general.

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We shall call

(F /F, E, ℓ, CK,V,Vbad, ϵ)

an initialΘ-data. From now on, we fix an initialΘ-data, and in the reminder of my talk, I will explain the following diagram which is

constructed from the given initial Θ-data (I only explain the constructions at non-archimedeanplaces which are the most important cases in the original form of IUT):

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Many constructions appeared in the above picture are not difficult to understand from geometry of coverings of curves. On the other hand, in IUT, we need to share information via various links between different Hodge theaters (or different universes) by using fundamental groups (via anabelian geometry), then we need some group-theoretical descriptions.

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§4: Construction of D-ΘNF-HT Reference

Section 4 of “S. Mochizuki, Inter-universal Teichm¨uller theory I:

Construction of Hodge theaters. Publ. Res. Inst. Math. Sci. 57(2021), 3–207.”

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In the section, I explain the right-hand side:

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D-prime-strips We put

Dv def=

{ πtp1 (Xlogv ) (or Btp(Xlogv )0), ifv∈Vbad π´et1(X−→v) (or B´et(−→Xv)0), ifv∈Vgood where B()0 denotes the subcategory of the Galois category B() consisting of connected objects, and −→Xvdef= X−→K×KKv is determined by (CK, ϵ) via the picture in the next page:

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Let Ddef= {Dv}v∈V. Then we put D>def

= {D>,v}v∈V (=D),

where D>,v=Dv for all v∈V.

Let F def= F×/{±1},Vj,j∈F, a copy ofV, andDj

def= {Dvj}vj∈Vj. Then we put

DJ

def= {Dj}jJ, where J def= F. Moreover, we put

Ddef= π´et1(CK) (or B´et(CK)0).

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-symmetry (on cusps)

For every v∈V, recall the following commutative diagram:

Xv def= XK×KKv −−−−→Z/ℓZ Xv

def= XK×KKv

±1



y ±1y

Cv def= CK×KKv

degree

−−−−−→ Cv

def= CK×KKv. We put

LabCuspv def= the set of non-zerocusps of Cv. Then we have

LabCuspv F (def= F× /{±1}) ={ℓ, ℓ1, . . . ,2,1,−2, . . . ,−ℓ}, where ϵv 7→the image of1 in F and def= (ℓ1)/2.

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Moreover, we put

LabCuspK def= the set of non-zero cusps of CK. Then there exists a natural bijection

LabCuspv LabCuspK, v∈V,

via the natural homomorphismCv →CK. This means that the sets {LabCuspv}v∈V

can be managed by LabCuspK via the above bijections.

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On the other hand, since CK is a “K-core”, we have Aut(CK),→Gal(K/Fmod).

Moreover, we define a subgroup

Autϵ(CK)def= Aut(CK) s.t.σ(ϵ) =ϵ}.

Let E[ℓ](F)↠Q(∼=F) be the quotient determined by the Galois ´etale covering XK →XK. Then we have the following

LabCuspK (

{QOXK} \ {OXK}) /{±1}

(

Q\ {0})

/{±1}. Thus, we obtain an exact sequence

1Autϵ(CK)Aut(CK)Aut(Q)/1}(=F)1.

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This means

Aut(CK)/Autϵ(CK)=F.

By using anabelian geometry, we have a group-theoretical version of the above isomorphism:

Aut(D)/Autϵ(D) F

and Aut(D)/Autϵ(D)is a sub-quotient of the Galois group of the extension of number fields Gal(K/Fmod). Moreover, we obtain the following action (=⊠-symmetry arising from arithmetic):

Aut(D)/Autϵ(D)↷LabCuspK (=F ↷ F).

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(Model) D-NF-bridge Let v∈V. We put

ϕNF,v :Dv → D

induced by Xv →Cv →CK ifv∈Vbad and X−→v →Cv →CK if v∈Vgood. We put (as a poly-morphism (i.e., a set of morphisms))

ϕNFv def= Autϵ(D)◦ϕNF,v Aut(Dv) :Dv→ D. Recall that Vj,j∈F, a copy of V. We write

Dj def

= {Dvj}vj∈Vj, where Dvj =Dv. Let

ϕNF1 def= NFv

1 }v1∈V1 :D1→ D be the poly-morphism determined by ϕNFv

1 ,v1V1.

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Since Autϵ(D)·ϕNF1 =ϕNF1 (i.e., stable under the action of Autϵ(D) by definition), we obtain an action of F Aut(D)/Autϵ(D)on ϕNF1 . Moreover, we put

ϕNFj def= j·ϕNF1 :Dj → D, j F. We shall call the poly-morphism

ϕNF def= NFj }j∈F

:DJ (orD)def= {Dj}j∈F

→ D

the(model) D-NF-bridge (recallJ =F).

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(Model) D-Θ-bridge We put

D>

def= {D>,v}v∈V, where D>,v =Dv.

Let v∈Vbad. Then we have the following morphism ϕeΘv

j :Dvj (=Dv)1

↠Gal(Kv/Kv)⃝→ D2 v, j F,

where 1is the natural surjection π1tp(Xv)↠Gal(Kv/Kv). Let us explain

2.

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Recall the tempered coverings whose special fibers are as following:

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Note that{ej1, . . . , ej}j∈F

areKv -rational points ofXv. Then we define

2 to be “the Galois section determined by a point of {ej1, . . . , ej}j∈F

” (roughly speaking,{ej1, . . . , ej}j∈F

is a finite approximation of evaluation points explained in §2 and the Galois sections contains the informations of values of Θv explained in §2). Then we have information about values of theta functions.

We put (as a poly-morphism) ϕΘv

j

def= Aut(D>,v)◦ϕeΘv

jAut(Dvj) :Dvj → D>,v, j F. On the other hand, let v∈Vgood. We put (as a full poly-isomorphism)

ϕΘv

j :Dvj(=Dv)→ D >,v(=Dv), j F.

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Moreover, for global case, we put ϕΘj def= Θv

j}vj∈Vj :Dj

def= {Dvj}vj∈Vj → D>

def= {D>,v}v∈V. Then we have

ϕΘ def= Θj}jJdef=F

:DJ (orD)def= {Dj}jJ → D>

and shall call ϕΘ the(model)D-Θ-bridge.

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Summary

We have D-ΘNF-HT as following

D> def

= {D>,v}v∈V ϕΘ

← DJ def

= {Dj}jJdef=F

def= {{Dvj}vj∈Vj}jJ ϕNF

→ D and, for each j∈J, the maps of sets of cusps (asF-torsors)

ϕLCj :LabCusp(D) LabCusp(Dj) LabCusp(D>), ϵ7→j.

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§5: Construction of Dell-HT Reference

Section 5 and Section 6 of “S. Mochizuki, Inter-universal Teichm¨uller theory I: Construction of Hodge theaters. Publ. Res. Inst. Math. Sci. 57 (2021), 3–207.”

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In this section, we explain the left-hand side:

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-symmetry (on cusps) For each v∈V, we put

LabCusp±v def= the set of cusps of Xv. Then we have the natural action of Gal(Xv/Cv)=1} on

LabCusp±v F={ℓ, . . . ,1,0,1, . . . ,−ℓ}. On the other hand, we put

LabCusp±Kdef= the set of cusps of XK.

Then we may manage the sets of cusps {LabCusp±v}v∈V via the natural bijection induced by Xv →XK:

LabCusp±v LabCusp±K, v∈V.

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We put

D def= {D,v}v∈V,

where D,v =Dv. Note that LabCusp±v can be mono-anabelian reconstructed from D,v. On the other hand, we put

D⊚±def= π´et1(XK) (or B´et(XK)0).

Note that LabCusp±K can be mono-anabelian reconstructed fromD⊚±. Then we obtain a group-theoretical version of the above bijection of cusps:

LabCusp±(D) LabCusp±(D⊚±).

We may identify LabCusp±(D) with LabCusp±K(D⊚±) via the above bijection. Moreover, there is a natural action

AutK(XK)(=F⋊± def= F1})↷LabCusp±K(=F).

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In fact, the above action can be expressed group-theoretically. We put Aut±(D⊚±)def= ker(Aut(D⊚±)F),

where the homomorphism is determined by the quotient E[ℓ](F)→Q and Q∼=F is introduced in§4, and put

Autcusp(D⊚±)Aut(D⊚±)

the subgroup of automorphisms which fix the cusps of XK. Then we have the following action (=⊞-symmetry arising from geometry):

(AutK(XK)=)Aut±(D⊚±)/Autcusp(D⊚±)↷LabCusp±K(D⊚±) which can be regarded as

F⋊± ↷ F

by using ϵ.

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(Model) D±-bridge

Let T =LabCusp±K(D⊚±)={ℓ, . . . ,1,0,−1, . . . ,−ℓ} with action of F⋊± andVt,t∈T, a copy ofV. We put a poly-isomorphism

ϕΘv±

t :Dvt(=Dv)

Aut+(D⊚±)-orbit

D,v,

where, roughly speaking, Aut+(D⊚±)Aut±(D⊚±) is the subgroup such that σ(“positive labels”) =“positive labels” for all σ∈Aut+(D⊚±).

Moreover, we put

ϕΘt± def= Θvt±}vt∈Vt :Dt

def= {Dvt}vt∈Vt → Ddef= {D,v}v∈V. Then we shall put

ϕΘ±±def= Θt±}tT :DT (or D±)def= {Dt}tT → D

and callϕΘ±± the(model)D±-bridge.

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(Model) Dell-bridge For v∈V, we put

ϕΘ,vell :Dv→ D⊚±

the morphism determined by the natural morphism Xv →Xv →XK if v∈Vbad and X−→v →Xv →XK ifv∈Vgood. Write

ϕΘvell

0

def= Autcusp(D⊚±)◦ϕΘ,vell Aut+(Dv0) :Dv0 → D⊚±, ϕΘ0ell def= Θvell

0 }v0∈V0 :D0

def= {Dv0}v0∈V0 → D⊚±. Note that since

t∈T (=F)F⋊± =Aut±(D⊚±)/Autcusp(D⊚±)↷ϕΘ0ell, we put

ϕΘtell def= t·ϕΘ0ell :Dtdef

= {Dvt}vt∈Vt → D⊚±.

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We shall put

ϕΘ±ell def= Θtell}tT :DT def

= {Dt}tT → D⊚±

and call ϕΘ±ell the(model)Dell-bridge.

Summary

We have D±ell-HT as following:

Ddef= {D,v}v∈V ϕΘ±±

← DT

def= {Dt}tT(∼=F)

def= {{Dvt}vt∈Vt}tT ϕΘell±

→ D⊚±. Note that we do not have any information about theta functions by the definition of D-Θ±ell-HT. To obtain that, we need to “glue”D-Θ±ell-HT with D-ΘNF-HT.

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§6: Θ±ellNF-Hodge theaters Reference

Section 6 of “S. Mochizuki, Inter-universal Teichm¨uller theory I:

Construction of Hodge theaters. Publ. Res. Inst. Math. Sci. 57(2021), 3–207.”

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D-ΘNF-Hodge theater We shall call

D>

ϕΘ

DJ ϕNF

D

a D-ΘNF-Hodge theater if it is “isomorphic” to (i.e., poly-isomorphisms

D> → D >,DJ → D J,D→ D satisfy certain compatible conditions) D>

ϕΘ

← DJ ϕNF

→ D.

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D±ell-Hodge theater We shall call

DϕΘ

±±

DT ϕΘell±

D⊚±

a D-Θ±ell-Hodge theater if it is “isomorphic” to (i.e., poly-isomorphisms

D → D ,DT → D T,D⊚± ∼→ D⊚± satisfy certain compatible conditions)

DϕΘ

±±

← DT ϕΘell±

→ D⊚±.

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D±ellNF-Hodge theater

We put the following identifications (i.e., “a gluing”) (T\ {0})/1}=J.

Then we can construct a D-Θ-bridge

ϕΘ[ϕΘ±±] :DJ

def= {Dt}tJ D>

from any D±-bridgeϕΘ±± :DT D. Then we shall call a triple (D±ell-HT,D-ΘNF-HT,ϕΘ[ϕΘ±±]=ϕΘ)

D±ellNF-Hodge theater.

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Then we obtain the following diagram mentioned above.

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Frobenioids

Let Dbe aD-prime-strip (i.e., a fundamental group or (the subcategory of connected objects of) a Galois category). We put

F

the Frobenioid whose base isD. Roughly speaking, Fis a category over D whose objects are “rational functions” on objects (i.e., coverings) of D.

Θ±ellNF-Hodge theater

We obtain the following which is called Θ±ellNF-Hodge theater, and whose base is D±ellNF-Hodge theater:

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Thank you for the attention!

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