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1
vote
is there a way to solve the following equation?
Maybe I do not understand the question, but since the vector of eigenvalues of $C(y)$ is $A^t y$, computing the spectral decomposition for $y = e_1, \dotsc, e_m$ (the basis vectors) gives you $A.$ The …
1
vote
Accepted
A Bessel integral
I suspect your fears are justified. You are basically trying to integrate the integrand from $0$ to $a$ (since, as you have noted, and Mathematica confirms, the integral from $0$ has a closed form) Yo …
4
votes
Is this statement about the real edge space of a graph known or trivial?
The set of $u$-cycles of a graph $G$ spans $\mathbb{R}^{E(G)}$ if and only if $G$ consists has only the single vertex $u$. For otherwise, the space spanned by the cycles in the graph is a vector space …
24
votes
Accepted
Vanishing of $\hat{A}$ genus and positive scalar curvature
After thinking about this question for two hours, my belief is strengthened that there cannot be a proof of the vanishing of the A-genus
for spin manifolds with positive scalar curvature without usin …
17
votes
Wanted: example of a non-algebraic singularity
The main question of the PI has been beautifully answered by Moret-Bailly, but not the
secondary question arisen from his expectation: "I would expect that it should be possible to write down a power …
3
votes
Components of an exceptional divisor
I had a minor thought. Did you ever look the paper Multiplicity of the special fiber of blowups by Corso,Polini, Vasconcelos.
In particular, they give an upper bound on the number (I realize that …
8
votes
Is there a non-Hopfian lacunary hyperbolic group?
Since Henry also discusses the property of being equationally Noetherian, I think the following observation is worth posting. And it is too long for a comment, so I post it as an answer.
The observa …
1
vote
Show that holomorphic functions are infinitely differentiable without complex analysis
This already got asked, and I answered it: Is there an integration free proof (or heuristic) that once differentiable implies twice differentiable for complex functions?
3
votes
When is a class function on a group G (finite abelian) into the rational numbers Q an elemen...
A necessary criterion is that, if $g \in G$ has order $m$, and $k$ is relatively prime to $m$, then $f(g) = f(g^k)$.
Proof: Let $\rho$ be a rational representation. Let the eigenvalues of $\rho(g)$ …
12
votes
Accepted
When are constructible points closed?
Let $X$ be locally noetherian. Then $\{x\}$ is constructible if and only if $\{x\}$ is locally closed (for non-noetherian schemes the notion of constructibility is more complicated and all kind of ter …
13
votes
Accepted
What is the limit of the "knight" distance on finer and finer chessboards?
Let $(x,y)$ and $(x+2a,y+a)$ be points in space. Then clearly the distance between these two points is $a$. Therefore, the unit ball around 0 must contain the octagon with vertices $(2,1)$, $(1,2)$, $ …
6
votes
Show that holomorphic functions are infinitely differentiable without complex analysis
It can be instructive to consider the Cauchy-Riemann equations as an elliptic system of partial differential equations. For elliptic systems, there is a $C^\infty$-regularity result similar to the one …
3
votes
Accepted
Giambelli and Porteous Formula
William Fulton, Flags, Schubert polynomials, degeneracy loci, and determinantal formulas, Duke Math J. 65 (1992) 381--420
3
votes
Show that holomorphic functions are infinitely differentiable without complex analysis
Edit: Avoided by $C^2$ by using integrals.
Edit 2: I believe that the following manipulations of $U,V$ work out right...
If $f$ satisfies the Cauchy-Riemann equations, then write $f(x,y) = (u(x,y), …
3
votes
Accepted
Some Pcf Theory
Hi. I know this is over a year late, but I the proof you're looking at matches that given on page 61-62 of "Cardinal Arithmetic". The argument can be finished along the following lines:
(1) First, …