Creating vast beautiful mansions from the becoming of nothing

 L’espace d’un homme film source

Alexander Grothendieck, who signed his works in French “Alexandre” but otherwise kept the spelling of his German-Jewish heritage, passed away Thursday in southwestern France.

Today we mourn his passing, and try to describe some of his vision.

Part of the story of this amazing mathematician is that in 1970 he renounced his central position at the Institut des Hautes Études Scientifiques (IHES) in Paris, and made himself so remote shortly after formally retiring from the University of Montpellier in 1988 that not even family and friends could track him. He boycotted his 1966 Fields Medal ceremony in Moscow to protest the Red Army’s presence in eastern Europe, and declined the Crafoord Prize in 1988.

As captured by this obituary, he had left to seek a society kinder and more just than the ones that killed his father at Auschwitz and convicted him in 1977 of violating a French law dating to 1945 against feeding and sheltering an unregistered alien. More will be told of this story as his voluminous writings from the hinterland are being read. But between 1945 and 1970 he published mathematics of unparalleled sweep and power, conveying escalations of abstraction to the solution of concrete problems, and this is the part we wish to appreciate.

## The Space Outside the Cave

One of humanity’s greatest intellectual tropes is Plato’s “Allegory of the Cave,” which likens what we apprehend through our senses to shadows of forms projected on a wall by a dimly-lit fire. The forms hail from an outside world whose light is blinding to one prisoner unchained and led out of the cave. Although Plato speaking through Socrates addressed all of reality, let us just imagine this outside world as Euclidean space, in which the Platonic solids shimmer in their ideal forms. Then what Grothendieck perceived when he was led into the light is the following:

The outside of the cave is another cave.

In this outer cave the focal point is ${0}$, zero. This zero is the only solution to the equation ${x = 0}$. It is also the only solution in that cave to the equation ${x^2 = 0}$. Likewise ${x^3 = 0}$, ${x^4 = 0}$, and so on. These are different equations, but each has only the same single root in the Euclidean space of the outer cave. We can add the words “with different multiplicities,” but what difference do they make to the objects from which we draw our solution?

Perhaps this ${0}$ is but a projection along a beam of elements in a higher space that can furnish different solution structures to these different equations. What Grothendieck regarded as needed for “truly natural methods in geometry,” as related by Jacob Murre quoting a lecture by Grothendieck in 1959, is the employment of nilpotents, that is, elements ${e}$ such that the sequence ${e,e^2,e^3,e^4,\dots}$ eventually gives zero. Such elements can be as simple as ${2}$ modulo ${4}$ or the matrix ${[^0_0{\,}^1_0]}$, but organizing them is what unchains us from the single zero.

## Space and Syntax

In moving essays written by Grothendieck’s friend and colleague Pierre Cartier for the 40th and 50th anniversaries of the IHES, coinciding with Grothendieck’s 70th and 80th birthdays, Cartier did not shrink from invoking Albert Einstein for intellectual comparison. Nor did Grothendieck, as the latter essay relates regarding the approach to space.

Einstein famously derived the core of his physical theories by working out all the logical consequences of the visions in his thought experiments. One of them is that there is no focal point of space. Nor is space a pre-existing entity, as Isaac Newton had posited, but rather space emerges from relational properties of its contents. A manifold is not made by its points, and it need not be determined by the locally Euclidean structure near any one point, but rather by how open sets around points mesh together. But in math, when we have no matter, what can we take as the content that drives the structure?

In executing mathematics we can approach contents only via definitions and formulas and proofs, which are pieces of syntax. Plato was aware of this. In his “Allegory of the Divided Line,” which immediately precedes the “Cave” passage in his Republic, Plato divided the mathematical world internally in the same ratio by which he divided it from the world of sense experience. Mathematical Platonists distinguish themselves from formalists by affirming reality beyond formulas and proofs, which can seem like chains on the intellect. However, all schools can alike acclaim the way major advances in the 20th Century came from treating syntax as objects. One example comes clearest in Leon Henkin’s proof of Kurt Gödel’s Completeness Theorem by employing logical statements as elements of the constructed model.

Cartier’s 1998 essay picks right up from this. Consider a model ${M}$ assigning a truth value ${v_M(P) \in \{0,1\}}$ to every proposition ${P}$ in some Boolean algebra ${{\cal A}}$. Associate to ${P}$ the set ${[P]}$ of all models ${M}$ that make ${v_M(P) = 1}$. These sets obey the rules:

$\displaystyle [P \wedge Q] = [P] \cap [Q],\qquad[P \vee Q] = [P] \cup [Q].$

Thus our “points” ${P}$ correspond to special subsets of the space ${{\cal M}}$ of models. The sets of ${M}$ can be generalized to sets of valuation functions ${v: {\cal A} \longrightarrow [0,1]}$ obeying ${v(\bot) = 0}$, ${v(\top) = 1}$, and for all ${P,Q \in {\cal A}}$,

$\displaystyle v(P) + v(Q) = v(P \wedge Q) + v(P \vee Q),$

giving ${[P] = \{v: v(P) = 1\}}$. In algebraic geometry there is a similar relation between points and special sets of functions giving equations that they solve. The upshot is that if we can identify the “special” property so that other sets besides our original ${[P]}$‘s have it, then from those sets we can harvest more “points.”

## What Are the Points?

Suppose we have a system ${B}$ of equations ${f_1(X) = 0,\dots,f_s(X) = 0}$, where ${X}$ ranges over some space ${S}$. Consider all objects of the form

$\displaystyle f = g_1 \cdot f_1 + g_2 \cdot f_2 + \cdots + g_s \cdot f_s,$

where the multipliers ${g_i(X)}$ are arbitrary functions, including constants. Then any common solution ${a}$ to the equations also makes ${f(a)= 0}$. The set ${I_B}$ of such functions can be regarded as the “algebraic consequences” of ${B}$. ${I_B}$ is clearly closed under addition and under multiplication by arbitrary elements, so it forms an ideal in the function space.

Now consider any point ${a = (a_1,\dots,a_n)}$ in an ${n}$-dimensional Euclidean space ${R^n}$. It is the unique solution to the simple system ${B = B_a}$ of equations

$\displaystyle f_1 = x_1 - a_1,\; f_2 = x_2 - a_2,\;\dots\; f_n = x_n - a_n,$

of course setting each ${f_i = 0}$. The ideal ${I_B}$ is then maximal in the space ${S = R[x_1,\dots,x_n]}$ of polynomials over ${R}$, meaning that ${I_B \neq S}$ but for any other ideal ${J}$,

$\displaystyle I_B \subseteq J \subseteq S \implies J = I_B \vee J = S.$

Every maximal ideal ${I}$ is prime, meaning that if a product ${gh}$ belongs to ${I}$, then either ${g \in I}$ or ${h \in I}$. If neither were in ${I}$, then the ideal ${J = \{f + rg : f \in I, r \in R\}}$ would properly contain ${I}$ and hence have to be all of ${S}$, whereupon we could find a scalar ${r}$ such that ${f + rg = 1}$. But then ${h = 1\cdot h = (f+rg)h = fh + r(gh)}$ would belong to ${I}$ after all.

The concepts of ideal and prime and maximal can be applied even in simpler spaces ${S}$ such as the set of integers. Every integer ${a}$ generates the ideal ${I_a}$ of multiples of ${a}$. If ${a}$ factors properly as ${bc}$, then ${b \notin I_a}$ and ${c \notin I_a}$, so ${I_a}$ is not prime, while ${I_a \subset I_b}$ and ${I_a \subset I_c}$, so ${I_a}$ is not maximal. But when ${a}$ is prime, ${I_a}$ is both prime and maximal, and these are the only prime or maximal ideals of ${\mathbb{Z}}$. For other spaces such as our polynomials over ${R}$, however, the concepts of prime and maximal do not coincide. Grothendieck culminated a long list of people who realized that while maximality is the “pointy” property, primality is the “special” one.

With respect to our original points ${a \in R^n}$, for any ideal ${I}$ we can identify the set

$\displaystyle V(I) = \{a: f(a) = 0 \text{ for each } f \in I\},$

called the algebraic set or variety determined by ${I}$. If ${I = I_B}$, then it is enough that ${V_I}$ be the common solution set of the ${f_i}$ in ${B}$. There is something analogous to the above example of Boolean valuations going on, except that the set operations are flipped around:

$\displaystyle V(I \cap J) = V(I) \cup V(J),\qquad V(I \oplus J) = V(I_B) \cap V(I_{B'}).$

Here ${I \oplus J}$ means the ideal closure of ${I \cup J}$, and equals ${I_{B \cup C}}$ when ${I = I_B}$ and ${J = I_C}$. The product ${IJ = \{fg: f \in I \wedge g \in J\} = I_{BC}}$ also gives ${V(IJ) = V(I) \cup V(J)}$.

We must skip over some wonderful finiteness theorems by David Hilbert and his students—and over distinctions such as the “base ring” ${R}$ being a field that is/is-not algebraically closed and projective versus affine space—to say only that algebraic sets are primitively defined in first-order arithmetic and hence are “neat” in many senses. Specially neat are those ${V}$ that cannot be written as ${V = V(I) \cup V(J)}$ in a nontrivial way, that is without one of ${V(I)}$ or ${V(J)}$ being all of ${V}$. Then we can’t have ${V = V(IJ)}$ or ${V = V(I \cap J)}$ in a nontrivial manner, and exactly what this means is that the ideal ${I(V)}$ of all functions that vanish on ${V}$ is prime. Such a ${V}$ is called irreducible, and sometimes the term “variety” is still reserved for this case. In an abstract but natural way, irreducible varieties of all dimensions up to ${n-1}$ can be made to behave like points. To quote Cartier on “the meaning of the word scheme” (his emphasis):

“One must, of course, understand that the space Grothendieck associated with an algebraic variety is not the set of its own points, but the set of its irreducible subvarieties.”

## Syntax Goes it Alone

To see where the nilpotents come in, and how Grothendieck unchained us not only from zero but from Euclidean points overall, we can begin by perceiving how evaluating a function is like doing long division with remainder. With respect to any ideal ${I}$ of ${S}$, the relation ${f - g \in I}$ is an equivalence relation, and allows us to write the quotient ${S/I}$. For our Euclidean point ${a}$, evaluating ${f(a)}$ can be achieved syntactically by reducing ${f}$ modulo (the ideal ${I_a}$ generated by) ${B_a}$. Taking ${f}$ modulo ${x_1 - a_1}$ by long division works out the same as substituting ${x_1 := a_1}$, and the same goes iteratively with the other elements of ${B_a}$.

The reduction process works for any ideal ${I}$ and gives a unique result ${f'}$, provided a special kind of basis ${B}$ giving ${I = I_B}$ named after Wolfgang Gröbner is used for the iterated long division. Thus the “evaluation” ${f' = f \bmod{I}}$ is well-defined for any ideal ${I}$ and can be carried out by an algorithm that first expands any initial set of generators for ${I}$ into a Gröbner basis. Alas all known algorithms have doubly exponential worst-case time complexity, perhaps unavoidably since deciding whether ${f \bmod{I} = 0}$ is complete for exponential space even when ${f}$ is linear and the initial generators for ${I}$ have constant degree. Nevertheless, these algorithms are run all the time for important equation-solving applications, and impress on us this philosophical fact:

We can do richer kinds of evaluation in the space delineated by our syntax than in the external Euclidean space.

This holds even when we return to our simple equations ${f_1 = x}$, ${f_2 = x^2}$ with ${n = 1}$, only one variable. The ideal ${I_1 = I_{\{x\}}}$ is prime—indeed maximal—but ${I_2 = I_{\{x^2\}}}$ is not prime. Also ${I_2 = I_1\cdot I_1}$, and the ideal ${I_3}$ generated by ${x^3}$ equals ${I_1\cdot I_2}$. Nevertheless, when we reduce a polynomial like ${f = x^3 + x^2 + x + 1}$ modulo these respective ideals, we get different results.

Thus we can dispense with the original points, even the origin in ${R^n}$. However, we would still like to preserve our primitive idea of “evaluation” in some kind of external space. How can we do this, and in what kind of space?

## Laying Out Spaces

We cannot squeeze these answers out of our Euclidean space. We can interpret a quotient ${S/I}$ as endowing ${V(I)}$ with coordinates as a space in its own right, but that only works up to the prime ideals. Once we connected irreducible varieties to prime ideals, that much was one-and-done. We can’t get multiple function values ${f \bmod{I_1}}$, ${f \bmod{I_2}}$, ${f \bmod{I_3}}$ out of our single, irreducible zero. There is no “square root of zero” different from zero. To go further, Grothendieck drew inspiration from how multiple-valued complex functions such as square-root and ${\log}$ can still be treated as holomorphic, by “snipping” and then “layering” ${\mathbb{C}}$ to fan out their branches.

For square-root, let us snip the non-negative real axis out of the complex plane. This leaves an open subset ${O}$, on which every ${z \in O}$ has a unique square root ${s_+(z)}$ with positive real part. The function ${s_{+}}$ is analytic on ${O}$, as is the other branch ${s_{-}}$. To get them to coexist as a single entity with the essence of being holomorphic, however, requires a way of building “layers” on ${O}$, and on other open subsets as needed to cover the part of ${\mathbb{C}}$ that was snipped out.

Here is where the edifices become tall and the abstraction too steep to cover in a single post. Considering the infinitely-branching ${\log}$ function on one hand, and infinitely many degrees of equations on the other, we can expect that infinite structures will be employed. Indeed, Grothendieck built them above every open subset ${O}$ in a “glued-together” manner. We cannot even easily resort to our usual sign-off to “see the paper for details.”

Yet we can say that the structures carry the idea of “becoming” points in ${O}$ via the concepts of fibres and sheaves, and that nilpotent elements are employed. Einstein’s relational foundation is actuated by what Grothendieck termed his “relative view” of defining morphisms between representations as regulated by category theory, rather than defining stand-alone objects. The category of sheaves is abstracted to topos theory, by which the Greek word for “place” supplants the original idea of “point.” His French word étale described a flat sea as “spreading” like these layerings. It further reflects his heritage by deriving from a German word estal meaning “place,” whereby it also connotes spreading out goods in layers in a stall that can be one of many spread out over a marketplace.

## Personal Spaces

All this became massive, so much that Grothendieck’s manuscripts before and after leaving IHES spread to hundreds and thousands of pages, as well as his personal memoir in the 1980s. Indeed, as related by Winfried Scharlau in a 2008 article for the AMS Notices, some of Grothendieck’s colleagues believed that he tired at the prospect of climbing his own mountains.

We can “morph” the description of Grothendieck’s “rising sea” approach in an essay by Colin McLarty to say that Grothendieck preferred to harness surveyors, engineers, and dam-builders so he could float to the top on rising waters, rather than do the ascent by “hammer and chisel.” He decried Pierre Deligne’s 1974 completion of their program of proving the famous conjectures by André Weil by methods he and others felt were not “morally right” on account of bypassing Grothendieck’s still-open “standard conjectures.”

A 2004 article by Allyn Jackson reproduces a cartooned abstract by Grothendieck for a colloquium in 1971 by which he warned that doing his lecture “in black-and-white detail for Springer Lecture Notes would likely take 400–500 pages,” ending by writing that from “a life-distancing logical delirium” it was “high time to change course.” Today’s mathematical community has in two years still barely touched a similarly-motivated though procedurally different theory erected by Shinichi Mochizuki on foundations named for Oswald Teichmüller, despite its “mere” 512 pages in first drafts.

## Where it Touches Complexity

So what can all this mean for us who work in what Grothendieck described as a “mansion” in which “the windows and blinds are all closed,” while he was one of those “whose spontaneous and joyful vocation it has been to be ceaseless building new mansions”? At least he did not call our dwelling a cave. However, in complexity theory we have it worse than Plato’s cave-prisoners in not merely missing the blinding world outside, but sensing its impact as a negative image in our present ignorance of lower bounds.

Much of complexity theory translates naturally to questions about polynomials over finite fields. This goes not only to ${\mathbb{F}_2}$ for Boolean functions but also to ${\mathbb{F}_p}$ and ${\mathbb{F}_{p^k}}$ for higher primes ${p}$ and ${k > 1}$, which in turn yield questions about Boolean solutions to equations over these fields. There are possible advances to be had by improving the partial correspondence to problems in zero characteristic. The larger program out of the Weil conjectures seeks to transfer geometry to positive characteristic. Can we see how its further development might allow us to extract combinatorial results needed to put bounds on complexity?

Polynomials modulo composite numbers give us a more immediate frontier, one represented by the complexity class ${\mathsf{ACC^0}}$, whose nonuniform version was only recently separated from nondeterministic exponential time. These polynomials behave badly in manners stemming from nilpotent elements in the rings ${\mathbb{Z}_m}$ for composite ${m}$. Can we somehow supply “extra points” and valuations to raise their structure toward that of polynomials over finite fields, and thus at least achieve bounds known for polynomials modulo primes?

A third example, my favorite and most immediate for the theme of this post, concerns the famous ${\Omega(n\log n)}$ lower bound of Volker Strassen and Walter Baur on the size of arithmetic circuits computing some natural families of polynomial functions ${f(x_1,\dots,x_n)}$ in zero characteristic. Its proof, as we recounted in 2010, turns on a property of geometric degree that pertains only to affine Euclidean space (or to its projective cousin). It employs the ideal ${J_f}$ generated by

$\displaystyle y_1 - \frac{\partial f}{\partial x_1},\;\dots,\; y_n - \frac{\partial f}{\partial x_n},$

where the “mapping variables” ${y_i}$ ensure the ideal is prime since the graphs of all mappings are irreducible varieties. Unfortunately, the highest geometric degree ${\gamma}$ attainable for ${J_f}$ when ${f}$ has ordinary total degree ${d+1}$ is ${d^n}$, whose logarithm for ${d = \mathsf{poly}(n)}$ gives the known lower bound. This bound however holds for simple functions such as ${f = x_1^n + x_2^n + \cdots x_n^n}$, while ${\log \gamma}$ comes nowhere close to the exponential lower bounds we conjecture for functions such as the ${n \times n}$ permanent.

Higher algebraic geometry has yielded notions of “algebraic degree” that can go one or more exponential orders higher, typically ${d^{2^n}}$. If the Strassen-Baur technique could be transferred to their higher spaces, then we could hope for strong lower bounds. The étale idea and related facets of algebraic geometry and representation theory also animate Ketan Mulmuley’s “Geometric Complexity Theory” programme. I once tried to find a combinatorial shortcut using a degree-like measure ${\mu}$ of counting “minimal monomials” in ideals, which we described here. It is striking that the determinant polynomials score zero on this measure, whereas the permanents score astronomically even for ${n=5}$, but as with the other degree measures ${\mu}$, there are counterexamples to ${\log\mu}$ being a circuit size lower bound.

## Open Problems

Can the answer to ${\mathsf{P}}$ versus ${\mathsf{NP}}$ be caught up in the “rising sea”? Or will it need something even stronger than “hammer and chisel”? What can we learn from his work? A sign of hope is that for all their heft and abstraction, his schemes can be programmed.

Our condolences to his relations and friends.

[“life-enrapturing”->”life-distancing”]

1. November 16, 2014 11:23 pm

brings to mind the distinction gowers made in an essay about theory-builders vs problem solvers. grothendieck was the unrivalled king of theory builders, erdos a king of problem solvers. see also impact of grothendieck’s program on tcs

November 17, 2014 6:29 am

Thank you for this beautiful tribute. I hesitate between Cantor, Hilbert and Einstein for an accurate comparison… Or maybe Galois… Poincaré… Turing… He’s really incomparable!

It’s a pity he had to spend the end of his life on his own for so many years. Hope he wasn’t too unhappy. Also, from what I’ve read of “Récoltes et semailles”, I find that essay very instructive in many respects – both mathematically and socially.

Glad to see algebraic geometry implemented on computer, though I wonder what’s expected of such an enterprise. I thought the QED Manifesto had failed… Maybe it’s just the first step before rewriting it all into Voevodsky’s univalent foundations…?

3. November 17, 2014 8:18 am

Very nice post, I would just like to note that everywhere else I’ve read that he spelled his name Alexander and not Alexandre. Also, it seems unsure that his mother, from whom he got his name, was of jewish origin or not.

4. November 17, 2014 10:47 am

Reblogged this on varasdemate and commented:

5. November 17, 2014 1:46 pm

R.I.P. A.G.

PS – nice presentation of Plato’s awareness. The passage in which Plato introduces his mystical “Nuptial Number” is said to be one of the most difficult passage in his writings. The Babylonian origin makes me wonder, `whatever inspired humans to project their souls onto the stars in the first place.’

6. November 17, 2014 6:57 pm

It’s nice to see a post that gives some picture of his work, as difficult as that is for a general-ish audience! An introduction to these ideas that covers less ground but is maybe even more gentle and gives a few more examples can be found in the Arithmetic Geometry chapter of the Princeton Companion to Mathematics: http://press.princeton.edu/chapters//gowers/gowers_IV_5.pdf.

7. November 17, 2014 9:26 pm

Is there an easy to understand explanation of why the permanent is so much harder to compute than the determinant.

• November 17, 2014 11:28 pm

Great question. Of course it’s hard to explain something that we haven’t yet proved :-), and in characteristic 2, it’s not. Before Valiant’s result in the mid-1970’s the absence of even competitive algorithms was thought mysterious. My best effort at a philosophical explanation is that when you write axioms that characterize each function, the ones for determinant “feel more natural.” Determinants are volumes.

Thanks, everyone above. I’m glad my sentence about Plato’s “awareness” seems OK—I considered adding that geometric diagrams were the “proofs” of his day. I have no clue on the “nuptial number”—it out-cryptics anything I’ve ever said… The Cartier 40th essay says that his mother was Jewish, and what Grothendieck himself wrote about his childhood attests to “heritage”; I used a French reference for “signed his works in French ‘Alexandre’ ” and here is an example. I’ve used Singular and would use SAGE for investigative research, not proofs per-se.

I wish I could have said more about the use of nilpotents in a simple manner. Thanks for suggesting the Gowers “Companion” which I own. The section by Jordan Ellenberg is nice as you say and indeed covers similar ground, but on nilpotents it makes only one remark on “quotient by the nilradical.” It does say more directly that schemes intend to bundle features for all base-rings together; I just kept the post agnostic about “R.” What disappointed me was that I couldn’t get a simple example of using nilpotents explicitly from Murre’s survey (nor the others) even though it furnishes the short quote I used about nilpotents and an even better longer one I didn’t:

However in spite of this restriction, a proof of this assertion remained (and – to my knowledge – remains up to now) entirely out of reach of the techniques and methods of the theory of algebraic varieties themselves, i.e. out of reach if we do not allow nilpotent elements to enter into the theory, and the above conjecture was actually only proved in 1959 by Grothendieck, using his theory of schemes.

This does reinforce the point I did make in the “Syntax Goes it Alone” section and the start of the next about stepping beyond prime ideals, but Murre’s next mention is “a closed subscheme defined by a nilpotent ideal.” With work I could have shown nilpotents within some quotient ${S/I}$, but it would have taken more work to connect that “extensionally” to the geometric picture for the non-prime ideals. Of course lots else besides Hilbert had to be left on the cutting-room floor, and I ducked away at the end from drawing a contrast to Teichmüller’s ardent Nazism.

• November 18, 2014 10:18 am

Ken Regan, that is a great answer! In a similar spirit — and for fun — for the remainder of 2014 I’m resolved to accompany my blog comments with concrete falsifiable predictions.

Here is a weak postulate relating to permanents, followed by a strong postulate:

Weak permanent postulate  Indistinguishably simulating a scattershot boson-sampling permanent distribution, by a classical computation, in the asymptotic limit of large photon-number, at the same entropy-cost as experimentally sampling that distribution (e.g., by the methods of arXiv:1305.4346), would not collapse the polynomial hierarchy.

Strong permanent postulate  Efficient, classical, indistinguishable simulations of scattershot boson-sampling permanent distributions, will be feasible in practice within a decade.

My personal confidence in the weak (resp. strong) postulate is fairly high (resp. moderately sanguine) … this is because ample complexity-theory wiggle-room is associated to various alternative definitions of “indistinguishable” … indeed, a TCS StackExchange question is in-the-works regarding these crucial definitional alternatives.

Obviously, no one knows whether these permanent-related postulates are true or false. What’s inarguable is that they’re great fun to think about, and highly instructive, equally for mathematicians, physicists, and engineers.

• November 18, 2014 1:46 pm

You are absolutely right about the Companion chapter: Ellenberg and you cover similar basics but then your goals seem to diverge. He focuses on how schemes show structure at a level “above” the particular ring. You focus on the nilpotents idea. It cannot be easy to pick one highlight when you want to give a glimpse of such vast mansions :).

• November 18, 2014 8:43 pm

Thanks, John and Sasho (and Serge below for the story in French). I also like the way Ellenberg identifies the “points” with homomorphisms from, say, ${S/I}$ into ${\mathbb{C}}$ (or into whatever is the “base ring” ${R}$), whereas I referenced the reduction map explicitly. Regarding the points, I found another source that just says the case ${I = (x^2)}$ with 1 variable makes the scheme for ${S/I}$ into a two-point space, with one point corresponding to the nilpotent element ${x}$ in this quotient. I would like to have more-vivid examples…

8. November 18, 2014 9:10 am

The list of references that Ken Regan and Dick Lipton provide is awesome! Here are a few more, that relate to Grothendieck’s celebrated question “What is a metre?”. Note: a shorter discussion of these references was posted to Shtetl Optimized, and presumably will appear there eventually.

—–

Scott wonders [in #37, referring to #13] “Would Grothendieck have accepted the Planck length as an answer to his question [‘what is a metre?’]? Or would that have involved an illegitimate intrusion of actual facts about physics?”

The work of Grothendieck’s IHES colleague, the physicist David Ruelle, points toward a friendly, sympathetic, interesting, ‘Bourbakian’, physics-friendly, and concrete answer to Scott’s question:

What is a metre? [in the spirit of Grothendieck/ Bourbaki/ IHES/ Ruelle]  A metre is a paradigmatic member of the interlocking mises en pratique that are associated to (what is called) “the new Systeme International (SI)”. More generally, a mathematically natural and physics-friendly 21st century extension of the Bourbaki program is to explicate the entirely of the new SI — both its microscope quantum dynamics and its macroscopic thermodynamics — in the natural language of algebraic dynamics and geometry.

A student-friendly course of reading might begin with Grothendieck’s IHES physicist-colleague David Ruelle’s recent book The Mathematician’s Brain (Princeton University Press, 2007), which contains considerable information of general interest to readers of Gödel’s Lost Letter and Shtetl Optimized.

Ruell’s material specifically relating to Grothendieck is pitched to non-specialist yet mathematically mature readers. This material includes Ruelle’s Chapter 6 “A glimpse into algebraic geometry and arithmetic”

“Stating things at the proper level of generality is an art. It is rewarded by obtaining a more natural and general theory, and also, very importantly, by bringing answers to questions that could be stated but not answered in the less general theory.”

and Ruelle’s Chapter 7 “A trip to Nancy with Alexander Grothendieck”

I must admit sympathy for the man [Grothendieck], not just admiration for the mathematician. People with moral preoccupations, and courage, are uncommon. And scientists do not rate better than average in this respect.[…]

In the case of Grothendieck [seeking post-IHES employment], he was nothing (not even having at the time French or any other citizenship). He was nobody’s responsibility; he was just an embarassment.

Understandably, some people would like to blame Grothendieck’s exclusion entirely on Grothendieck himself: he went crazy and left mathematics. But this does not fit with the known facts and their chronology.

Something shameful has taken place. And the disposal of Grothendieck will remain a disgrace in the history of twentieth-century mathematics.

These readings can be followed by Ruelle’s article “Is our mathematics natural? The case of equilibrium statistical mechanics” (Bull AMS, 1988). Then the real meat begins with (for example) Hansjorg Scherer and Benedetta Camarota’s recent survey “Quantum metrology triangle experiments: a status review” (2012).

The Grothendieck/Bourbaki/IHES/Ruelle perspective  A great many practical problems in quantum systems engineering — What is a metre? What is an ohm? What is a gravity wave? What is transmembrane ion transport? And broadly, what are the fundamental limits to the observation, simulation, and control of biological/medical/healing processes? — can be naturally and systematically explicated in the Bourbakian light of the Ruelle question (extended version): “Is our mathematics natural? The case of nonequilibrium statistical mechanics and transport theory.”

Two meanings of natural  Here natural is to be understood — as plausibly Ruelle intended — both in physicist-sense of “natural science” and the Grothendieck/Mac Lane/Eilenberg/Bourbaki sense of “natural mathematics.”

Bonus readings in Ruelle  Ruelle’s sympathetic discussion, in The Mathematician’s Brain, of Saharon Shelah’s (marvelous!) survey article “Logical dreams” (Bull AMS, 2003) also is commended to readers of Gödel’s Lost Letter and Shtetl Optimized.

• January 9, 2015 4:10 am

Another Grothendieck reference  To Ars Mathematica’s excellent discussion What Did Grothendieck Do?” I have contributed excerpts from the Grothendieck-era mathematical roadmap The Mathematical Sciences; a Report (1968, National Academy of Sciences pub. #1681). Not every roadmap committee gets it right … this one did!

November 18, 2014 8:17 pm

Most instructive article (in French):

http://www.reporterre.net/spip.php?article6576

Where we learn – among other things – that AG grew his passion for ecology in the US, though he *never* ceased to do mathematics (cosmology…), and that he’d never have become a mathematician, had he been younger…