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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 …
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 …
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, …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …