2009 September 10th - questionsEdit

Navigation: 20 Questions home | view answers | next week

Contents 1 2009 September 10th - questions 1.1 1 Scott 1.2 2 Critch 1.3 3 Yuhao 1.4 4 Yael 1.5 5 Mike 1.6 6 Darsh 1.7 7 Jakob 1.8 8 Andrew D 1.9 9 Anton 1.10 10 Pablo 1.11 11 Andrew D again

1 ScottEdit

In R², you can tile the plane with hexagons. However any closed trivalent graph has a face that’s smaller than a hexagon. You can tile R³ with vertex-truncated octahedrons. Say we have a “generic” closed finite cell-complex (every edge has 3 incident faces, every vertex has 4 incident edges). Is there something “smaller” than a vertex-truncated octahedron (or the other polytopes that give generic tilings)?

VIEW/POST ANSWERS

2 CritchEdit

Is there a space with trivial homology, non trivial homotopy? (Anton: isn’t there a result that say that first nontrivial homology and homotopy agree?)

VIEW/POST ANSWERS

3 YuhaoEdit

Let A be an abelian category, that might not have enough injectives? Can you embed into another abelian category with enough injectives? Is there a universal way? e.g. finite abelian groups embeds into -modules e.g. coherent sheaves embeds into quasi-coherent sheaves (Anton: the Freyd embedding theorem says every abelian category embeds in -mod. But this doesn’t help universality.)

VIEW/POST ANSWERS

4 YaelEdit

Out(G) = Aut(G)/Inn(G). Is there a nice description of cosets, beyond that they’re cosets?

VIEW/POST ANSWERS

5 MikeEdit

X a banach space, convex. If X is infinite dimensional, what extra conditions guarantee that f is continuous?

VIEW/POST ANSWERS

6 DarshEdit

Take a triangle in R² with coordinates at rational points. Can we find the smallest denominator point in the interior? (Take the LCM of the denominators of the coordinates.) (You can do the 1D version using continued fractions.)

VIEW/POST ANSWERS

7 JakobEdit

Take a “sparse” (every vertex has reasonably small degree) graph. Consider a maximal independent set for the graph (a maximal set of disconnected vertices). Can we make a new graph, with vertices the set, and whatever edges we like, that is ``as topologically similar to the original graph as possible? (What does this mean?)

VIEW/POST ANSWERS

8 Andrew DEdit

Consider the sequence x₀ = 0,x₁ = 1,x_{n + 2} = ax_{n + 1} + bx_n, generalizing the Fibonacci sequence. Fix p a prime. If k is minimal such that p | x_k and p | x_l implies k | l, then v_p(x_nk) = v_p(x_k) + v_p(n). (Here v_p(z) is the power of p dividing z.) Is there some framework that makes this sort of result obvious? Andrew only knows strange proofs.

VIEW/POST ANSWERS

9 AntonEdit

Take I = [0,1), the half open interval. Do there exist topological spaces X and Y, with X and Y not homeomorphic, but and are homeomorphic? E.g., if instead I = [0,1], the closed interval, you can take X=mickey mouse=disc with two discs removed, Y=cross-eyed frog=disk with two linked bands glued on the boundary.

VIEW/POST ANSWERS

10 PabloEdit

x^x^x^x … converges if . E.g. with , this converges to 2. Given a sequence (a_i), when does the “power tower sequence” converge?

VIEW/POST ANSWERS

11 Andrew D againEdit

Can you define the set of all primes with a finitely-axiomatized first-order theory?

Given a first-order theory T, let . You can get all prime powers with the field axioms. Is there some T so S(T) is the set of primes?

VIEW/POST ANSWERS