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4 votes
Accepted

Derivatives of infinite order

In the case of a single variable, see for example this article concerning the limit $\lim_{n\to\infty}f^{(n)}(x)$ for a smooth function $f:\mathbb R\to\mathbb R$. Also, if $f:\mathbb R\to\mathbb R$ i …
Joonas Ilmavirta's user avatar
14 votes
Accepted

How nilpotent is the ring of stable homotopy groups of spheres?

Let $\alpha \in \pi_s(S^0)$ for $s > 0$ be an element of positive degree in the stable stems. Then $\alpha$ has positive-dimensional filtration in the Adams-Novikov spectral sequence: in other words, …
Akhil Mathew's user avatar
  • 25.6k
17 votes
Accepted

Equivariant version of Morse theory

The answer to your question is yes, of course. The theory has been around at least since the late 60s! See Wasserman's paper A Wasserman. Equivariant differential topology, Topology 1969; 8(2):12 …
Vidit Nanda's user avatar
  • 15.5k
4 votes
Accepted

Is an non-singualr invertable ergodic transformation on a measure space isomorphic to its in...

You cannot obtain an example from odometers because they have discrete point spectrum. It is not true in the topological setting- look at example 7.4.19 from http://books.google.ca/books/about/An_In …
Nishant Chandgotia's user avatar
2 votes

Relations between Multizeta Values

The relation is $\zeta(4,1)=2\zeta(5)-\zeta(2)\zeta(3)$. It can be found, for example, in http://projecteuclid.org/DPubS?verb=Display&version=1.0&service=UI&handle=euclid.pjm/1102636166&page=record (M …
Zurab Silagadze's user avatar
2 votes

Jordan-Hölder theorem for subfactors?

A proof for the class $\mathcal{C}$ group-subgroup subfactors: First of all, by the Galois correspondence, an intermediate subfactor $R^G \subset P \subset R^H$ is given by an intermediate subg …
Sebastien Palcoux's user avatar
0 votes

The category of subfactors extending the category of groups?

This is an artificial answer, I'm looking for something more natural. In this paper, T. Teruya introduced the notion of normal intermediate subfactors, generalizing exactly the notion of normal sub …
Sebastien Palcoux's user avatar
3 votes
Accepted

Ranks of higher incidence matrices of designs

For the generalization in the first direction, the $p$-rank of the incidence matrix $N$ of an $S(2,k,v)$ is lower bounded by the dimension of the Steinberg module: $$\operatorname{rank}_2(N)(\operato …
Yuichiro Fujiwara's user avatar
4 votes

Dynamic of $SL_2(\mathbb{Z}$) on $\mathbb{C}^2$

The dynamics are ergodic with respect to Lebesgue measure. See G. Hedlund, Fuchsian groups and mixtures, Ann. of Math. Volume 40, Number 2 (1939) 370-383, available here. If you prefer somethi …
Vidit Nanda's user avatar
  • 15.5k
28 votes

Illustrating Edward Nelson's Worldview with Nonstandard Models of Arithmetic

If you are really satisfied with a model only of the theory $Q$, then you should be prepared for a bad situation, for this is an extremely weak theory. In fact one can make a computable model simply b …
Joel David Hamkins's user avatar
8 votes

the spectrum of the Laplacian and Dirac operator on $S^3$

The spectrum of the Dirac operator on spheres was computed by Christian Bär: http://projecteuclid.org/DPubS?verb=Display&version=1.0&service=UI&handle=euclid.jmsj/1226499694&page=record See also th …
Stephan Mescher's user avatar
33 votes
Accepted

Why Cohen-Macaulay rings have become important in commutative algebra?

I think there are many reasons. Here are a few. Practical reasons Cohen-Macaulay rings are just plain easier to work with. Computations in local cohomology For example, any number of computatio …
Karl Schwede's user avatar
  • 20.5k
2 votes

Converse to Chow's theorem in sub-riemannian geometry

I think the correct reference should be Theorem 1 in Tadashi NAGANO, Linear differential systems with singularities and an application to transitive Lie algebras, J. Math. Soc. Japan Volume 18, Numbe …
Dario's user avatar
  • 381
10 votes
Accepted

Where was it first stated that there are no 4-transitive finite groups other than symmetric,...

In Pacific Journal of Math 4 (1954), pp 219-226, Marshall Hall, Jr. writes in the paper "On a theorem of Jordan" that: In 1872, Jordan showed that a finite quadruply transitive group in which only …
Arturo Magidin's user avatar