Grothendieck | neverendingbooks

Posts Tagged ‘Grothendieck’

noncommutative F_un geometry (2)

Friday, October 17th, 2008

Last time we tried to generalize the Connes-Consani approach to commutative algebraic geometry over the field with one element $\mathbb{F}_1$ to the noncommutative world by considering covariant functors

$N~:~\wis{groups} \rightarrow \wis{sets}$

which over $\C$ resp. $\Z$ become visible by a complex (resp. integral) algebra having suitable universal properties.

However, we didn’t specify what we meant by a complex noncommutative variety (resp. an integral noncommutative scheme). In particular, we claimed that the $\mathbb{F}_1$ -’points’ associated to the functor

$D~:~\wis{groups} \rightarrow \wis{sets} \qquad G \mapsto G_2 \times G_3$ (here G_n denotes all elements of order of )

were precisely the modular dessins d’enfants of Grothendieck, but didn’t give details. We’ll try to do this now.

For algebras over a field we follow the definition, due to Kontsevich and Soibelman, of so called “noncommutative thin schemes”. Actually, the thinness-condition is implicit in both Soule’s-approach as that of Connes and Consani : we do not consider R-points in general, but only those of rings R which are finite and flat over our basering (or field).

So, what is a noncommutative thin scheme anyway? Well, its a covariant functor (commuting with finite projective limits)

$\mathbb{X}~:~\wis{Alg}^{fd}_k \rightarrow \wis{sets}$

from finite-dimensional (possibly noncommutative) -algebras to sets. Now, the usual dual-space operator gives an anti-equivalence of categories

$\wis{Alg}^{fd}_k \leftrightarrow \wis{Coalg}^{fd}_k \qquad A=C^* \leftrightarrow C=A^*$

so a thin scheme can also be viewed as a contra-variant functor (commuting with finite direct limits)

$\mathbb{X}~:~\wis{Coalg}^{fd}_k \rightarrow \wis{Sets}$

In particular, we are interested to associated to any {tex]k[/tex]-algebra its representation functor :

$\wis{rep}(A)~:~\wis{Coalg}^{fd}_k \rightarrow \wis{Sets} \qquad C \mapsto Alg_k(A,C^*)$

This may look strange at first sight, but C^* is a finite dimensional algebra and any -dimensional representation of is an algebra map $A \rightarrow M_n(k)$ and we take to be the dual coalgebra of this image.

Kontsevich and Soibelman proved that every noncommutative thin scheme $\mathbb{X}$ is representable by a -coalgebra. That is, there exists a unique coalgebra $C_{\mathbb{X}}$ (which they call the coalgebra of ‘distributions’ of $\mathbb{X}$ ) such that for every finite dimensional -algebra we have

$\mathbb{X}(B) = Coalg_k(B^*,C_{\mathbb{X}})$

In the case of interest to us, that is for the functor $\wis{rep}(A)$ the coalgebra of distributions is Kostant’s dual coalgebra A^o . This is the not the full linear dual of but contains only those linear functionals on which factor through a finite dimensional quotient.

So? You’ve exchanged an algebra for some coalgebra A^o , but where’s the geometry in all this? Well, let’s look at the commutative case. Suppose $A= \C[X]$ is the coordinate ring of a smooth affine variety , then its dual coalgebra looks like

$\C[X]^o = \oplus_{x \in X} U(T_x(X))$

the direct sum of all universal (co)algebras of tangent spaces at points $x \in X$ . But how do we get the variety out of this? Well, any coalgebra has a coradical (being the sun of all simple subcoalgebras) and in the case just mentioned we have

$corad(\C[X]^o) = \oplus_{x \in X} \C e_x$

so every point corresponds to a unique simple component of the coradical. In the general case, the coradical of the dual coalgebra A^o is the direct sum of all simple finite dimensional representations of . That is, the direct summands of the coalgebra give us a noncommutative variety whose points are the simple representations, and the remainder of the coalgebra of distributions accounts for infinitesimal information on these points (as do the tangent spaces in the commutative case).

In fact, it was a surprise to me that one can describe the dual coalgebra quite explicitly, and that $A_{\infty}$ -structures make their appearance quite naturally. See this paper if you’re in for the details on this.

That settles the problem of what we mean by the noncommutative variety associated to a complex algebra. But what about the integral case? In the above, we used extensively the theory of Kostant-duality which works only for algebras over fields…

Well, not quite. In the case of $\Z$ (or more general, of Dedekind domains) one can repeat Kostant’s proof word for word provided one takes as the definition of the dual $\Z$ -coalgebra of an algebra (which is $\Z$ -torsion free)

$A^o = \{ f~:~A \rightarrow \Z~:~A/Ker(f)~\text{is finitely generated and torsion free}~\}$

(over general rings there may be also variants of this duality, as in Street’s book an Quantum groups). Probably lots of people have come up with this, but the only explicit reference I have is to the first paper I’ve ever written. So, also for algebras over $\Z$ we can define a suitable noncommutative integral scheme (the coradical approach accounts only for the maximal ideals rather than all primes, but somehow this is implicit in all approaches as we consider only thin schemes).

Fine! So, we can make sense of the noncommutative geometrical objects corresponding to the group-algebras $\C \Gamma$ and $\Z \Gamma$ where $\Gamma = PSL_2(\Z)$ is the modular group (the algebras corresponding to the $G \mapsto G_2 \times G_3$ -functor). But, what might be the points of the noncommutative scheme corresponding to $\mathbb{F}_1 \Gamma$ ???

Well, let’s continue the path cut out before. “Points” should correspond to finite dimensional “simple representations”. Hence, what are the finite dimensional simple $\mathbb{F}_1$ -representations of $\Gamma$ ? (Or, for that matter, of any group )

Here we come back to Javier’s post on this : a finite dimensional $\mathbb{F}_1$ -vectorspace is a finite set. A $\Gamma$ -representation on this set (of n-elements) is a group-morphism

$\Gamma \rightarrow GL_n(\mathbb{F}_1) = S_n$

hence it gives a permutation representation of $\Gamma$ on this set. But then, if finite dimensional $\mathbb{F}_1$ -representations of $\Gamma$ are the finite permutation representations, then the simple ones are the transitive permutation representations. That is, the points of the noncommutative scheme corresponding to $\mathbb{F}_1 \Gamma$ are the conjugacy classes of subgroups $H \subset \Gamma$ such that $\Gamma/H$ is finite. But these are exactly the modular dessins d’enfants introduced by Grothendieck as I explained a while back elsewhere (see for example this post and others in the same series).

Tags: arxiv, coalgebras, Connes, Dedekind, geometry, google, Grothendieck, groups, Kontsevich, modular, noncommutative, permutation representation, representations
Posted in geometry | No Comments »

noncommutative F_un geometry (1)

Monday, October 13th, 2008

It is perhaps surprising that Alain Connes and Katia Consani, two icons of noncommutative geometry, restrict themselves to define commutative algebraic geometry over $\mathbb{F}_1$ , the field with one element.

My guess of why they stop there is as good as anyone’s. Perhaps they felt that there is already enough noncommutativity in Soule’s gadget-approach (the algebra $\mathcal{A}_X$ as in this post may very well be noncommutative). Perhaps they were only interested in the Bost-Connes system which can be entirely encoded in their commutative $\mathbb{F}_1$ -geometry. Perhaps they felt unsure as to what the noncommutative scheme of an affine noncommutative algebra might be. Perhaps …

Remains the fact that their approach screams for a noncommutative extension. Their basic object is a covariant functor

$N~:~\wis{abelian} \rightarrow \wis{sets} \qquad A \mapsto N(A)$

from finite abelian groups to sets, together with additional data to the effect that there is a unique minimal integral scheme associated to . In a series of posts on the Connes-Consani paper (starting here) I took some care of getting rid of all scheme-lingo and rephrasing everything entirely into algebras. But then, this set-up can be extended verbatim to noncommuative $\mathbb{F}_1$ -geometry, which should start from a covariant functor

$N~:~\wis{groups} \rightarrow \wis{sets}$

from all finite groups to sets. Let’s recall quickly what the additional info should be making this functor a noncommutative (affine) F_un scheme :

There should be a finitely generated $\C$ -algebra together with a natural transformation (the ‘evaluation’)

$e~:~N \rightarrow \wis{maxi}(R) \qquad N(G) \mapsto Hom_{\C-alg}(R, \C G)$

(both and the group-algebra $\C G$ may be noncommutative). The pair $(N, \wis{maxi}(R))$ is then called a gadget and there is an obvious notion of ‘morphism’ between gadgets.

The crucial extra ingredient is an affine $\Z$ -algebra (possibly noncommutative) such that is a subfunctor of $\wis{mini}(S)~:~G \mapsto Hom_{\Z-alg}(S,\Z G)$ together with the following universal property :

any affine $\Z$ -algebra having a gadget-morphism $~(N,\wis{maxi}(R)) \rightarrow (\wis{mini}(T),\wis{maxi}(T \otimes_{\Z} \C))$ comes from a $\Z$ -algebra morphism $T \rightarrow S$ . (If this sounds too cryptic for you, please read the series on C-C mentioned before).

So, there is no problem in defining noncommutative affine F_un-schemes. However, as with any generalization, this only makes sense provided (a) we get something new and (b) we have interesting examples, not covered by the restricted theory.

At first sight we do not get something new as in the only example we did in the C-C-series (the forgetful functor) it is easy to prove (using the same proof as given in this post) that the forgetful-functor $\wis{groups} \rightarrow \wis{sets}$ still has as its integral form the integral torus $\Z[x,x^{-1}]$ . However, both theories quickly diverge beyond this example.

For example, consider the functor

$\wis{groups} \rightarrow \wis{sets} \qquad G \mapsto G \times G$

Then, if we restrict to abelian finite groups $\wis{abelian}$ it is easy to see (again by a similar argument) that the two-dimensional integer torus $\Z[x,y,x^{-1},y^{-1}]$ is the correct integral form. However, this algebra cannot be the correct form for the functor on the category of all finite groups as any $\Z$ -algebra map $\phi~:~\Z[x,y,x^{-1},y^{-1}] \rightarrow \Z G$ determines (and is determined by) a pair of commuting units in $\Z G$ , so the above functor can not be a subfunctor if we allow non-Abelian groups.

But then, perhaps there isn’t a minimal integral $\Z$ -form for this functor? Well, yes there is. Take the free group in two letters (that is, all words in noncommuting $x,y,x^{-1}$ and $y^{-1}$ satisfying only the trivial cancellation laws between a letter and its inverse), then the corresponding integral group-algebra $\Z \mathcal{F}_2$ does the trick.

Again, the proof-strategy is the same. Given a gadget-morphism we have an algebra map $f~:~T \mapsto \C \mathcal{F}_2$ and we have to show, using the universal property that the image of is contained in the integral group-algebra $\Z \mathcal{F}_2$ . Take a generator of then the degree of the image f(z) is bounded say by and we can always find a subgroup $H \subset \mathcal{F}_2$ such that $\mathcal{F}_2/H$ is a fnite group and the quotient map $\C \mathcal{F}_2 \rightarrow \C \mathcal{F}_2/H$ is injective on the subspace spanned by all words of degree strictly less than d+1 . Then, the usual diagram-chase finishes the proof.

What makes this work is that the free group $\mathcal{F}_2$ has ‘enough’ subgroups of finite index, a property it shares with many interesting discrete groups. Whence the blurb-message :

if the integers $\Z$ see a discrete group $\Gamma$ , then the field $\mathbb{F}_1$ sees its profinite completion $\hat{\Gamma} = \underset{\leftarrow}{lim}~\Gamma/ H$

So, yes, we get something new by extending the Connes-Consani approach to the noncommutative world, but do we have interesting examples? As “interesting” is a subjective qualification, we’d better invoke the authority-argument.

Alexander Grothendieck (sitting on the right, manifestly not disputing a vacant chair with Jean-Pierre Serre, drinking on the left (a marvelous picture taken by F. Hirzebruch in 1958)) was pushing the idea that profinite completions of arithmetical groups were useful in the study of the absolute Galois group $Gal(\overline{\mathbb{Q}}/\mathbb{Q})$ , via his theory of dessins d’enfants (children;s drawings).

In a previous life, I’ve written a series of posts on dessins d’enfants, so I’ll restrict here to the basics. A smooth projective $\overline{\mathbb{Q}}$ -curve has a Belyi-map $X \rightarrow \mathbb{P}^1_{\overline{\mathbb{Q}}}$ ramified only in three points $\{ 0,1,\infty \}$ . The “drawing” corresponding to is a bipartite graph, drawn on the Riemann surface $X_{\C}$ obtained by lifting the unit interval [0,1] to . As the absolute Galois group acts on all such curves (and hence on their corresponding drawings), the action of it on these dessins d’enfants may give us a way into the multiple mysteries of the absolute Galois group.

In his “Esquisse d’un programme” ( Sketch of a program if you prefer to read it in English) he writes :

“C’est ainsi que mon attention s’est portée vers ce que j’ai appelé depuis la “géométrie algêbrique anabélienne”, dont le point de départ est justement une étude (pour le moment limitée à la caractéristique zéro) de l’action de groupe de Galois “absolus” (notamment les groupes $Gal(\overline{K}/K)$ , ou est une extension de type fini du corps premier) sur des groupes fondamentaux géométriques (profinis) de variétés algébriques (définies sur ), et plus particulièrement (rompant avec une tradition bien enracinée) des groupes fondamentaux qui sont trés éloignés des groupes abéliens (et que pour cette raison je nomme “anabéliens”). Parmi ces groupes, et trés proche du groupe $\hat{\pi}_{0,3}$ , il y a le compactifié profini du groupe modulaire $SL_2(\mathbb{Z})$ , dont le quotient par le centre $\pm 1$ contient le précédent comme sous-groupe de congruence mod 2, et peut s’interpréter d’ailleurs comme groupe “cartographique” orienté, savoir celui qui classifie les cartes orientées triangulées (i.e. celles dont les faces des triangles ou des monogones).”

and a bit further, he writes :

“L’élément de structure de $SL_2(\mathbb{Z})$ qui me fascine avant tout, est bien sur l’action extérieure du groupe de Galois $Gal(\overline{\Q}/\Q)$ sur le compactifié profini. Par le théorème de Bielyi, prenant les compactifiés profinis de sous-groupes d’indice fini de $SL_2(\mathbb{Z})$ , et l’action extérieure induite (quitte à passer également à un sous-groupe overt de $Gal(\overline{\Q},\Q)$ ), on trouve essentiellement les groupes fondamentaux de toutes les courbes algébriques définis sur des corps de nombres , et l’action extérieure de $Gal(\overline{K}/K)$ dessus.”

So, is there a noncommutative affine variety over $\mathbb{F}_1$ of which the unique minimal integral model is the integral group algebra of the modular group $\Z \Gamma$ (with $\Gamma = PSL_2(\Z)$ ? Yes, here it is

$N_{\Gamma}~:~\wis{groups} \rightarrow \wis{sets} \qquad G \mapsto G_2 \times G_3$

where G_n is the set of all elements of order in . The reason behind this is that the modular group is the free group product $C_2 \ast C_3$ .

Fine, you may say, but all this is just algebra. Where is the noncommutative complex variety or the noncommutative integral scheme in all this? Well, we can introduce them too but as this post is already 1300 words long, I’ll better leave this for another time. In case you cannot stop thinking about it, here’s the short answer.

The complex noncommutative variety has as its ‘points’ all finite dimensional simple complex representations of the modular group, and the ‘points’ of the noncommutative $\mathbb{F}_1$ -scheme are exactly the (modular) dessins d’enfants…

Tags: Connes, Galois, geometry, Grothendieck, groups, modular, noncommutative, profinite, representations, Riemann
Posted in geometry | No Comments »

This week at F_un Mathematics (1)

Saturday, October 11th, 2008

In case you haven’t noticed it yet : I’m not living here anymore.

My blogging is (at least for the moment) transfered to the F_un Mathematics blog which some prefer to call the “ceci n’est pas un corps”-blog, which is very fine with me.

Javier gave a talk at MPI on Soule’s approach to algebraic geometry over the elusive field with one element $\mathbb{F}_1$ and wrote two posts about it The skeleton of Soule’s F_un geometry and Gadgets a la Soule. The rough idea being that a variety over the field with one element only acquires flesh after a base extension to $\Z$ and to cyclotomic integers.

I did some posts on a related (but conceptually somewhat easier) approach due to Alain Connes and Katia Consani. I’ve tried to explain their construction at the level of (mature) undergraduate students. So far, there are three posts part1, part2 and part3. Probably there is one more session to come in which I will explain why they need functors to graded sets.

In the weeks to come we plan to post about applications of this F_un-geometry to noncommutative geometry (the Bost-Connes system) and Grothendieck’s anabelian geometry (the theory of dessins d’enfant). I’ll try to leave a short account of the main posts here, but clearly you are invited to feed your feedreader this.

Perhaps I’ll return here for a week mid november to do some old-fashioned vacation blogging. I have to admit I did underestimate Numeo.fr. Rumours have it that our place is connected wirelessly to the web…

Tags: anabelian, blogging, Connes, geometry, Grothendieck, noncommutative, wordpress
Posted in groups | No Comments »

Connes-Consani for undergraduates (3)

Friday, October 10th, 2008

A quick recap of last time. We are trying to make sense of affine varieties over the elusive field with one element $\mathbb{F}_1$ , which by Grothendieck’s scheme-philosophy should determine a functor

$\wis{nano}(N)~:~\wis{abelian} \rightarrow \wis{sets} \qquad A \mapsto N(A)$

from finite Abelian groups to sets, typically giving pretty small sets N(A) . Using the Fun mantra that $\Z$ should be an algebra over $\mathbb{F}_1$ any $\mathbb{F}_1$ -variety determines an integral scheme by extension of scalars, as well as a complex variety (by extending further to $\C$ ). We have already connected the complex variety with the original functor into a gadget that is a couple $~(\wis{nano}(N),\wis{maxi}(R))$ where is the coordinate ring of a complex affine variety having the property that every element of can be realized as a $\C A$ -point of . Ringtheoretically this simply means that to every element $x \in N(A)$ there is an algebra map $N_x~:~R \rightarrow \C A$ .

Today we will determine which gadgets determine an integral scheme, and do so uniquely, and call them the sought for affine schemes over $\mathbb{F}_1$ .

Let’s begin with our example : $\wis{nano}(N) = \underline{\mathbb{G}}_m$ being the forgetful functor, that is for every finite Abelian group, then the complex algebra $R= \C[x,x^{-1}]$ partners up to form a gadget because to every element $a \in N(A)=A$ there is a natural algebra map $N_a~:~\C[x,x^{-1}] \rightarrow \C A$ defined by sending $x \mapsto e_a$ . Clearly, there is an obvious integral form of this complex algebra, namely $\Z[x,x^{-1}]$ but we have already seen that this algebra represents the mini-functor

$\wis{min}(\Z[x,x^{-1}])~:~\wis{abelian} \rightarrow \wis{sets} \qquad A \mapsto (\Z A)^*$

and that the group of units $(\Z A)^*$ of the integral group ring $\Z A$ usually is a lot bigger than . So, perhaps there is another less obvious $\Z$ -algebra doing a much better job at approximating ? That is, if we can formulate this more precisely…

In general, every $\Z$ -algebra defines a gadget $\wis{gadget}(S) = (\wis{mini}(S),\wis{maxi}(S \otimes_{\Z} \C))$ with the obvious (that is, extension of scalars) evaluation map

$\wis{mini}(S)(A) = Hom_{\Z-alg}(S, \Z A) \rightarrow Hom_{\C-alg}(S \otimes_{\Z} \C, \C A) = \wis{maxi}(S \otimes_{\Z} \C)(A)$

Right, so how might one express the fact that the integral affine scheme with integral algebra is the ‘best’ integral approximation of a gadget $~(\wis{nano}(N),\wis{maxi}(R))$ . Well, to begin its representing functor should at least contain the information given by , that is, $\wis{nano}(N)$ is a sub-functor of $\wis{mini}(T)$ (meaning that for every finite Abelian group we have a natural inclusion $N(A) \subset Hom_{\Z-alg}(T, \Z A)$ ). As to the “best”-part, we must express that all other candidates factor through . That is, suppose we have an integral algebra and a morphism of gadgets (as defined last time)

$f~:~(\wis{nano}(N),\wis{maxi}(R)) \rightarrow \wis{gadget}(S) = (\wis{mini}(S),\wis{maxi}(S \otimes_{\Z} \C))$

then there ought to be $\Z$ -algebra morphism $T \rightarrow S$ such that the above map factors through an induced gadget-map $\wis{gadget}(T) \rightarrow \wis{gadget}(S)$ .

Fine, but is this definition good enough in our trivial example? In other words, is the “obvious” integral ring $\Z[x,x^{-1}]$ the best integral choice for approximating the forgetful functor $N=\underline{\mathbb{G}}_m$ ? Well, take any finitely generated integral algebra , then saying that there is a morphism of gadgets from $~(\underline{\mathbb{G}}_m,\wis{maxi}(\C[x,x^{-1}])$ to $\wis{gadget}(S)$ means that there is a $\C$ -algebra map $\psi~:~S \otimes_{\Z} \C \rightarrow \C[x,x^{-1}]$ such that for every finite Abelian group we have a commuting diagram

$\xymatrix{A \ar[rr] \ar[d]_e & & Hom_{\Z-alg}(S, \Z A) \ar[d] \\ Hom_{\C-alg}(\C[x,x^{-1}],\C A) \ar[rr]^{- \circ \psi} & & Hom_{\C-alg}(S \otimes_{\Z} \C, \C A)}$

Here, is the natural evaluation map defined before sending a group-element $a \in A$ to the algebra map defined by $x \mapsto e_a$ and the vertical map on the right-hand side is extensions by scalars. From this data we must be able to show that the image of the algebra map

$\xymatrix{S \ar[r]^{i} & S \otimes_{\Z} \C \ar[r]^{\psi} & \C[x,x^{-1}]}$

is contained in the integral subalgebra $\Z[x,x^{-1}]$ . So, take any generator of then its image $\psi(z) \in \C[x,x^{-1}]$ is a Laurent polynomial of degree say (that is, $\psi(z) = c_{-d} x^{-d} + \hdots c_{-1} x^{-1} + c_0 + c_1 x + \hdots + c_d x^d$ with all coefficients a priori in $\C$ and we need to talk them into $\Z$ ).

Now comes the basic trick : take a cyclic group of order , then the above commuting diagram applied to the generator of (the evaluation of which is the natural projection map $\pi~:~\C[x.x^{-1}] \rightarrow \C[x,x^{-1}]/(x^N-1) = \C C_N$ ) gives us the commuting diagram

$\xymatrix{S \ar[r] \ar[d] & S \otimes_{\Z} \C \ar[r]^{\psi} & \C[x,x^{-1}] \ar[d]^{\pi} \\ \Z C_n = \frac{\Z[x,x^{-1}]}{(x^N-1)} \ar[rr]^j & & \frac{\C[x,x^{-1}]}{(x^N-1)}}$

where the horizontal map is the natural inclusion map. Tracing $z \in S$ along the diagram we see that indeed all coefficients of $\psi(z)$ have to be integers! Applying the same argument to the other generators of (possibly for varying values of N) we see that , indeed, $\psi(S) \subset \Z[x,x^{-1}]$ and hence that $\Z[x,x^{-1}]$ is the best integral approximation for $\underline{\mathbb{G}}_m$ .

That is, we have our first example of an affine variety over the field with one element $\mathbb{F}_1$ : $~(\underline{\mathbb{G}}_m,\wis{maxi}(\C[x,x^{-1}]) \rightarrow \wis{gadget}(\Z[x,x^{-1}])$ .

What makes this example work is that the infinite group $\Z$ (of which the complex group-algebra is the algebra $\C[x,x^{-1}]$ ) has enough finite Abelian group-quotients. In other words, $\mathbb{F}_1$ doesn’t see $\Z$ but rather its profinite completion $\hat{\Z} = \underset{\leftarrow} \Z/N\Z$ … (to be continued when we’ll consider noncommutative $\mathbb{F}_1$ -schemes)

In general, an affine $\mathbb{F}_1$ -scheme is a gadget with morphism of gadgets $~(\wis{nano}(N),\wis{maxi}(R)) \rightarrow \wis{gadget}(S)$ provided that the integral algebra is the best integral approximation in the sense made explicit before. This rounds up our first attempt to understand the Connes-Consani approach to define geometry over $\mathbb{F}_1$ apart from one important omission : we have only considered functors to $\wis{sets}$ , whereas it is crucial in the Connes-Consani paper to consider more generally functors to graded sets. In the final part of this series we’ll explain what that’s all about.

Tags: Connes, Consani, F_un, geometry, Grothendieck, groups, noncommutative, profinite
Posted in geometry | No Comments »

Connes-Consani for undergraduates (1)
Friday, October 3rd, 2008

A couple of weeks ago, Alain Connes and Katia Consani arXived their paper “On the notion of geometry over $\mathbb{F}_1$ “. Their subtle definition is phrased entirely in Grothendieck’s scheme-theoretic language of representable functors and may be somewhat hard to get through if you only had a few years of mathematics.

I’ll try to give the essence of their definition of an affine scheme over $\mathbb{F}_1$ (and illustrate it with an example) in a couple of posts. All you need to know is what a finite Abelian group is (if you know what a cyclic group is that’ll be enough) and what a commutative algebra is. If you already know what a functor and a natural transformation is, that would be great, but we’ll deal with all that abstract nonsense when we’ll need it.

So take two finite Abelian groups A and B, then a group-morphism is just a map $f~:~A \rightarrow B$ preserving the group-data. That is, f sends the unit element of A to that of B and f sends a product of two elements in A to the product of their images in B. For example, if is a cyclic group of order n with generator g and is a cyclic group of order m with generator h, then every groupmorphism from A to B is entirely determined by the image of g let’s say that this image is . But, as and the conditions on a group-morphism we must have that $h^{in} = (h^i)^n = 1$ and therefore m must divide i.n. This gives you all possible group-morphisms from A to B.

They are plenty of finite abelian groups and many group-morphisms between any pair of them and all this stuff we put into one giant sack and label it $\wis{abelian}$ . There is another, even bigger sack, which is even simpler to describe. It is labeled $\wis{sets}$ and contains all sets as well as all maps between two sets.

Right! Now what might be a map $F~:~\wis{abelian} \rightarrow \wis{sets}$ between these two sacks? Well, F should map any abelian group A to a set F(A) and any group-morphism $f~:~A \rightarrow B$ to a map between the corresponding sets $F(f)~:~F(A) \rightarrow F(B)$ and do all of this nicely. That is, F should send compositions of group-morphisms to compositions of the corresponding maps, and so on. If you take a pen and a piece of paper, you’re bound to come up with the exact definition of a functor (that’s what F is called).

You want an example? Well, let