bio  website  front.math.ucdavis.edu/… 

location  Cornell  
age  45  
visits  member for  5 years, 2 months 
seen  1 hour ago  
stats  profile views  10,173 
Math professor at Cornell,
PhD 1996
1h

answered  Unifying Geometry for Characteristic Classes 
19h

comment 
Companion to theoretical physics for working mathematicians
And yet there exists a Princeton Companion to Mathematics. 
Dec 22 
revised 
Is there a “good” reason that the universal central extension of $SL(2,\mathbb Z)$ is $Br_3$?
added 175 characters in body 
Dec 22 
asked  Is there a “good” reason that the universal central extension of $SL(2,\mathbb Z)$ is $Br_3$? 
Dec 20 
comment 
Does an arbitrary product of $f$ and $f^\dagger$ belong to a universal enveloping algebra of the Heisenberg algebra?
For somebody learning the subject, I think saying there's "no essential difference" between, e.g., a finitedimensional and infinitedimensional vector space is very confusing. "The representation theory is the same" is good and one should leave it at that. 
Dec 13 
comment 
Codimension in zero and positive characteristic
Is this a properness over $Spec\ \mathbb Z$ issue? 
Dec 13 
comment 
kostant partition function vs Haar measure
I would say "the Kostant partition function is the Fourier transform of the reciprocal of the Weyl denominator", in that the first is a function on the weight lattice of a torus and the second is a function on the torus itself. 
Dec 10 
revised 
Can a subset of the plane have nontrivial $H_2$ or $\pi_2$?
edited body 
Nov 26 
comment 
Is there an improvement for the SchurHorn inequalities for positive semidefinite matrices?
If your matrix is special then it's unlikely that SchurHorn gives the best possible results. 
Nov 26 
comment 
Is there an improvement for the SchurHorn inequalities for positive semidefinite matrices?
Consider the case $A$ real diagonal, and realize, "No." 
Nov 25 
comment 
Is there an improvement for the SchurHorn inequalities for positive semidefinite matrices?
Let $\vec v = \sum_j c_j \vec e_j$, where $\vec v$ is the $i$th vector in your orthonormal basis, and the $(e_j)$ are an orthonormal eigenbasis (with eigenvalues $(\lambda_i))$, and $\sum_i c_i^2=1$. Then $A_{ii} = \langle \vec v,A \vec v\rangle = \sum_i c_i^2 \lambda_i \in [\lambda_{min},\lambda_{max}]$. It's a little weird to credit Schur and Horn with this. 
Nov 25 
comment 
Is there an improvement for the SchurHorn inequalities for positive semidefinite matrices?
If you're asking what I think you're asking, the answer is "Very obviously yes," enough so that I wonder if I'm misunderstanding the question. 
Nov 25 
comment 
Is there an improvement for the SchurHorn inequalities for positive semidefinite matrices?
By "improvement", I assumed that the question was "if we impose more conditions, do we get a better result?" i.e. that the matrices were still Hermitian. Then my dumb trick says "no, we don't". As to the second comment: any orthonormal basis will work, giving you the same permutahedron. 
Nov 25 
comment 
Is there an improvement for the SchurHorn inequalities for positive semidefinite matrices?
For the first one, can't you just add a multiple of the identity to embed the usual problem into the semidefinite one, i.e. they're the same level of difficulty? 
Nov 24 
awarded  Nice Question 
Nov 23 
comment 
Determinant of the oriented adjacency matrix of a tree
I added an example to show a cut vertex in action; darij is right. 
Nov 23 
revised 
Determinant of the oriented adjacency matrix of a tree
added 209 characters in body 
Nov 23 
comment 
Determinant of the oriented adjacency matrix of a tree
I want to know whether it's $1$ or $1$. 
Nov 23 
asked  Determinant of the oriented adjacency matrix of a tree 
Nov 23 
comment 
Is there an “adjacency matrix” for weighted directed graphs that captures the weights?
It is unfortunate that the "lowtemperature" limit, which is very reasonable terminology, is also called the "tropical" limit (outside Japan, where the convention is reversed). 