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Search options answers only not deleted not community wiki created 2010-09-28 - 2011-09-28
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 …
Igor Rivin's user avatar
  • 96.4k
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 …
Igor Rivin's user avatar
  • 96.4k
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 …
David Eppstein's user avatar
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 …
Johannes Ebert's user avatar
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 …
Joël's user avatar
  • 26k
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 …
Karl Schwede's user avatar
  • 20.5k
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 …
Denis Osin's user avatar
  • 2,648
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?
Ryan Reich's user avatar
  • 7,273
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 $\rho(g)$ …
David E Speyer's user avatar
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 …
Torsten Wedhorn's user avatar
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 $(2,1)$, $(1,2)$, $ …
Will Sawin's user avatar
  • 148k
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 …
Anatoly Kochubei's user avatar
3 votes
Accepted

Giambelli and Porteous Formula

William Fulton, Flags, Schubert polynomials, degeneracy loci, and determinantal formulas, Duke Math J. 65 (1992) 381--420
Alexander Woo's user avatar
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), …
Christopher A. Wong's user avatar
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, …
Todd Eisworth's user avatar

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