# Favorite definitions, theorems, proofs, etc.?

1. May 23, 2005

### honestrosewater

After seeing infinite sets defined negatively, I liked seeing them defined as sets that are equivalent to one of their proper subsets. I always thought [URL [Broken] diagonal argument[/url] was cool.
Do you have a favorite definition, theorem, proof, bit of knowledge you found especially insightful or useful... ?

Last edited by a moderator: May 2, 2017
2. May 23, 2005

### matt grime

*cough* that'd be Dedekind infinite, which is not, strictly speaking, the same as infinite.

Surely the best results in mathematics are those that allow us to talk about continuity without reference to epsilon and delta: pointset topology

3. May 23, 2005

### honestrosewater

What, it said... eh, well, now I like it even more.

4. May 23, 2005

### matt grime

They are equivalent if we have the axiom of choice at hand, but otherwise not necessarily so, though I know of no examples of this. However that is not at all surprising given I know next to nothing about set theory.

5. May 23, 2005

### honestrosewater

Yeah, that's what it said.
I also find the cut theorem, I don't know, suspiciously obvious. Here's one statement from my favorite UKish logician Wilfrid Hodges: ("l=" means "semantically entails"- i.e. decision not proof)
If X is a finite set of formulae and p and q are formulae, then if (X l= p) and (X, p l= q), then (X l= q).
So p is unnecessary and cut out, but it makes me nervous and I have to remind myself of the proof.

So now I'm curious- how do you transform a doughnut into a teacup? Edit: Is it very complicated?

Last edited: May 23, 2005
6. May 23, 2005

### Haelfix

The proof of the Atiyah Singer index theorem is my personal favorite. A Beautiful, elegant, completely nontrivial result that is expressed simply in one line, and is perhaps the most powerful concept in the latter half of the twentieth century bringing together several fields of mathematics and physics.

7. May 23, 2005

### matt grime

Donut to teacup? That's easy to visualize. Imagine the donut is made of plasticine, then just push your thumb into its surface - that creates a hollow. Surely from there you can see how to continue to make the hollow the inside of the cup and the hole the handle? Of course a formula is going to be horrendous.

8. May 23, 2005

### honestrosewater

Oh, okay, I wasn't sure how you were allowed to change it, but I looked it up. You just aren't allowed to tear it. Because then two points that were arbitrarily close to each other aren't.?

9. May 23, 2005

### matt grime

Yep, deformations must be continuous, ie no ripping.

10. May 23, 2005

### mathwonk

haelfix, are you saying the several hundred page proof of Atiyah Singer, as in R.T. Seeley et.al., is your model of elegance? or the statement? (or atiyah's 2 - line summary)

this proof is presumably "just" a generalization of hirzebruch's argument for his version of the riemann roch theorem.

if it is indeed the proof you find elegant, would you be willing to summarize the argument for us?

as i recall these proofs are slick but rather unenlightening, establishing axioms for the index then checking the axioms, a la the earlier innovative argument by Washnitzer.

And what do you think of the heat equation approach of patodi? or was that the elegant one?

You are certainly in good company selecting this theorem, as Atiyah - Singer just won the Abel prize for it, as I am sure you know.

i like lefschetz's proof there cannot be a vector field on the sphere with no zeroes as follows; choose any point where there is a non zero vector and visualize a small circle around that point where the vectors are all going roughly the same direction. Then visualize those vectors emanating from the boundary circle, qua the boundary of the complement of that disc. You will eventually be able to see that they wind twice around the boundary of the complementary disc, hence there must be, counting with multiplicities, two zeroes outside that disc.

i also like newton's proof that every monotone function is "riemann" integrable, as found e.g. in comenetz' or apostol's calculus books, (or newton).

i also like baby versions of atiyah - singer, like riemann - roch (the original version by riemann and roch), as well as later incarnations by washnitzer, hirzebruch, fulton. i am still trying to absorb even the statement of grothendieck's version.

there is an english translation of riemann's works coming out which should still be interesting.

Last edited: May 24, 2005
11. May 24, 2005

### mathwonk

in the spirit of axiomatic arguments for "index theorems" here is a little such proof for hirzebruch riemann roch for plane curves.

the problem is to compute the dimension L(D) of the space of meromorphic functions on the curve, having pole divisor supported in the divisor D. The difficulty is that this is an analytic and not a topological invariant, hence cannot be computed by the usual method of degeneration to easier cases.

Hence the whole idea is to replace it by a related topological invariant, and compute that instead. that is called the hirzebruch riemann roch theorem HRR.

By sheaf theory one equates the number L(D) with the dimension of a "zeroth" cohomology group, and then because a curve has dimension one, there is another more mysterious first cohomology group whose dimension is i(D). then the "index" of D, chi(D) = L(D)-i(D). it is this which we propsoe to compute topologically.

step 1): show the difference chi(D) -chi(O), where O is the zero divisor, = degree(D) = number of points in the divisor D, surely a topological invariant. this is a trivial sheaf theoretic exact sequences count.

step 2) show chi(O) is a topolopgical invariant. by more trivial exact sequences [i.e. fund thm of linear algebra: dim source = dim image + dim kernel] one shows two things:

i) chi(O) = chi(X) is a linear deformation invariant, i.e. chi(X) depends only on the degree of the plane curve X.

ii) chi(XunionY) = chi(X) + chi(Y) - number of intersection points of X,Y.

Since any plane curve can be deformed linearly to a union of two plane curves of lower degree, so we can use induction to compute chi(O). And since a conic is not only isomorphic to a line, but also deforms to a union of two lines, meeting at one point, we have chi(line) = chi(line)+chi(line)-1, so chi(line) = 1, which starts the induction.

[the proof of i) is by a sort of rouche's principle, i.e. one computes the chi by intersection or "integrating" an object defined on the whole plane, over the curve, and this operation is invariant under deforming the curve.]

now we already have that chi(D) = chi(O) + deg(D) is a toplogical invariant, and to compute it, we finish by showing that chi(O) = 1-g, where g is the topologicsal genus.

To do that we only have to establish the same properties i) and ii) which hold for chi(O), also for the arithmetic genus 1-g.

One can show, again by degeneration, that the topological genus of a plane curve satisfies g = (1/2)(d-1)(d-2) where d is the degree. [idea: degenerate to a union of d lines and note that there are exactly this number of holes in the resulting figure, e.g. a triangle has one hole, so g = 1].

Hence i) above holds, and then the formula for 1-g can be shown by high school level arithmetic to satisfy relation ii) as well. (use induction.)

This proves HRR for plane curves.

To deduce the full RRT one must identify the term i(D). This last step exceeds what is provided by Atiyah - Hirzebruch or any index theorem, and requires a "vanishing result" or a "duality" result. Namely (Roch) i(D) = L(K-D) where K is the divisor of a differential form. Hence (Riemann) i(D) = 0, if deg(D) > deg(K) = 2g-2, so in that case L(D) = deg(D) + 1-g, is itself topological.

I love this topic. but notice all the proofs above are so slick they proceed without the need of understanding anything! as a student you may like this, but as a researcher it sems to make life harder, at least for pedestrians like me.

Notice though, if you hope to prove a theorem in "all" dimensions, or some other lofty setting, you may need a proof technique that is so formal, that you do not need to process mentally all the grubby details, i.e. one that does not require you to understand everything, but allows you to free yourself up to just calculate. Ironic, eh?

Last edited: May 24, 2005
12. May 24, 2005

### mathwonk

does this help anyone? if you always wanted to know how to prove the famous riemann roch theorem, or even to know what it says, i ask you to try to read the previous post. unlikely as it may seem, this was an actual attempt to render an esoteric subject accessible to more people.

Last edited: May 24, 2005
13. May 24, 2005

### fourier jr

yes it's always nice when that can be done. i also like the definition of a group. it seems to have just enough axioms to be useful. you'd be giving up a lot if any one of the axioms were thrown away. i also like the pigeonhole principle (or ramsey's theorem).

14. May 25, 2005

### mathwonk

continuity is amazing, like proving an equation f(x) = 0 must have a solution just beacuse there is a point a such that f(a) < 0 and a point b where f(b) > 0, and f is continuos on [a,b].

the higher dimensional versions of this are even more amazing, and these use stokes theorem, so i like that, and its use in differential topology in general.

like proving brouwers fixed point theorem, or the fundamental theorem of algebra, or that no smooth tangent vector fields on the sphere can be all non zero.

this type of topological argument by deforming to a more doable case, is what lies also behind the HRR argument above, and the Atiyah Singer index theorem. It goes on and on.

I like deformation theory also in algebraic geometry, the theory of moduli, wherein one viosualizes a whole family of geometry surfaces or curves as just points on another surface. i.e. one speaks of what it emans for a sequence of surfaces to converge to another one, and then tracks what happens to various invariants of the surface, like the homology.

One of the greatest inovations was the infinitesimal deformation theory of spencer and kodaira, developed by grothendieck, wherein one can define a computable tangent space to such a family of surfaces. it is always a cohomology group.

it turns out for example then tanegnt space to the family of all curves of genus g, at the curve X, is dual to the space of quadratic differentials on X! By RRT this has dimension 3g-3, (when g > 1), so the moduli space of all curves has that dimension, as Riemann knew by other methods.

15. May 27, 2005

### mathwonk

many people like the little arguemnt referred to as "steinitz exchange lemma" for proving the cardinality of all maximal independent collections of vectors is the same, in a finite dimenbsional vector space.

I have just found that argument in riemanns famous paper on abelian fucntions, for the case of a maximal collection of non bounding curves on a given surface, 14 years before steinitz' birth.

at the beginning of that same paper riemann "puts off until a future time, a treatment of [topology] in a manner entirely independent of measurement", which apparently never appeared, perhaps until hausdorff's treatment roughly 50 years later.

16. May 28, 2005

### mathwonk

the sheaf theoretic proof of the de rham theorem is rather amazing, i.e. sheaf theory plus poincares lemma. as in spivaks diff geom book vol 1.

17. May 29, 2005

### The Rev

You people make me feel like a prole for saying this, but I'm rather fond of the good ole Pythagorean Theorem.

$$\phi$$

The Rev

18. May 29, 2005

### mathwonk

i also like it. and the non linear version in harold jacobs book: i.e. imagine 2 - dimensional cutouts of the head of pythagoras, erected on each side of a right triangle. Then the sum of the areas of the two heads erected on the two legs of the triangle, equals the area of the head erected on the hypotenuse.

I.e. pythagoras says that if you erect squares on each side of a right triangle, then the sum of the areas of the two squares on the two legs, equals the area of the square on the hypotenuse, but it has nothing to do with the figures being squares.

i also like euler's (generalized) formula V-E+F = 2-2g, where g is the genus of the polyhedron, in fact that may be my favorite theorem of all time.

in fact it is closely related to andre weil's sheaf theoretic proof of the de rham theorem. i.e. eulers formula says that if you decompose a polyhedron into pieces, namely the faces, that have no geometry themselves, i.e. they are convex, then the geometry of the whole polyhedron (its genus) is determined by the way the faces fit together, i.e. how many edges and so on there are, in comparison to how many faces and vertices.

this is what weil's proof of de rham says: i.e. if you cover a manifold by locally trivial, e.g. convex open sets, where the differential equation dW = 0 always implies that W = dU, for some U, then the de rham cohomology of the manifold is entirely determined by the topological cech cohomology of the open cover.

I.e. the idea is that to understand the global structure of something, anything, you decompose it into pieces such that each piece has no interesting geometry. then the global structure is determined by how those trivial pieces fit together.

Last edited: May 29, 2005
19. May 29, 2005

### mathwonk

another theorem i like is bezout and its refinements. i.e. we know a polynomial in one variable of degree d has at most d roots, and if we count complex roots and multiple roots proeprly, it has "exactly" d roots.

then bezout asks how many common solutions two equations f = 0, g = 0 of two variables have? If we count complex solutions, and solutioons at infinity, and count multiple intersections of the two curves proeprly, then they have deg(f).deg(g) common roots.

but sometimes in higer dimensions mkroe inhtersting things happen: i.e. two surfaces in three space have usually a curve of intersection, but sometimes, three surfaces do too. Say the intersection of two surfaces breaks up into a union of two separate curves. Then pass a third surface so it completely contains oine of the curves but not the other. Then it meets the other in a finite number of points.

so the interscetion of the three surfaces is one curve plus a finite number of points. What can you say then about the number of points?

This arises in a classical problem called the rpoblem of appolonius. i.e. given 5 conics in the complex plane, how many conics are tangent everywhere they meet to all 5? The equation for a conic represents a point in the projective 5 space of all conics. And the set of those conics tangent to a given conic is a hypersurface in the space of all conics. Hence 5 such hypersurfaces, one for each given conic, should meet in a finite number of points, the set of conics meeting all 5 given conics tangentially.

but notice that any double line meets everything it meets "tangentially", i.e. doubly. so there is a whole surface of extraneous solutions to our problem of finding conics tangent to 5 given ones, the surface of double lines, one for each line in the plane. so how many actual conics meet 5 given ones tangentially?

the correct solution was given by chasles in the 19th century and possibly helped give rise to schuberts calculus of intersections and the refinement of counting "excess" intersections, as develoepd in the last century by Fulton, MacPherson, and others, see Fulton's Intersection theory.

Last edited: May 29, 2005
20. May 29, 2005

### mathwonk

i also always liked the theorem that a triangle inscribed in a semi circle is a right triangle.

and i love the refinement of pythagoras, the "law of cosines".

i.e. a^2 + b^2 = c^2 - 2ab cos(t).

this shows that the squared length of a side of a triangle is expressed as a sort of square: c^2 = a^2 + b^2 - 2ab cos(t) = (A-B).(A-B) where A.B = B.A = ab cos(t), and A,B are the direction vectors rerpesenting the two sides of the triangle with lengths a,b.

this ointroduces algebra into geometry in a very powerful way. too bad no one explained it to me this way in high school, where i stupidly thought the law of cosines just a useless pain in the neck to memorize.