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This tag is used if a reference is needed in a paper or textbook on a specific result.

9 votes

Basic question about polytope duals

This is false, even in dimension $3$. Take a regular icosahedron. Then wiggle the vertices a bit -- still get an icosahedron, but it is not regular. If you take the barycenters and the convex hull, yo …
Lev Borisov's user avatar
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0 votes

Examples of toric threefolds

If you are willing to work with smooth stacks, rather than smooth toric varieties (or alternatively consider toric varieties with quotient singularities) then it is definitely possible. Combinatoria …
Lev Borisov's user avatar
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3 votes

Why is the mirror of rigid Calabi-Yau threefold singularity theory?

There is a toric complete intersection example, rather ad hoc, in arXiv:alg-geom/9402002. This particular example is related to the situation when a decomposition of anticanonical class on toric Fano …
Lev Borisov's user avatar
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4 votes
Accepted

Number of solutions of linear homogenous Diophantine equation inside a box

The solution set $L$ is clearly a sublattice in $\mathbb Z^d$. If $a$ is fixed and $N$ grows, the number of lattice elements in a box grows approximately as $cN^{rank}$. So if you have your statement …
Lev Borisov's user avatar
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2 votes

Virasoro action on the elliptic cohomology

I would like to add that (cohomology of) chiral de Rham complex has to be viewed as the large Kahler limit of halftwisted theory. Indeed, it does not use the Kahler data on the CY manifold and does no …
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26 votes
1 answer
1k views

Real square roots of symmetric matrices

In joint work with Andreas Fischle (TU Dresden, Germany) and Patrizio Neff (U Essen, Germany) we needed to use the following statement: If $S$ is a real $n\times n$ matrix with $S^2$ symmetric, then t …
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6 votes
Accepted

binomial/factorial identity mod p

Sorry, don't know a reference, but here is a quick argument. If $M=p^ab+c$ with $0\leq c\leq p^a-1$, then $$(1+x)^M=(1+x)^{p^ab}(1+x)^c =(1+x^{p^a})^b(1+x)^c \mod p. $$ In turn, this equals $$ (1+bx …
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3 votes

Three questions concerning lattice points on sphere surfaces

It is not really my area, but the answer to part 2 is that you are looking for the smallest $N$ such that there are exactly $8n-4$ ways of writing it in the form $a^2+b^2$. If such $N$ is even but i …
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7 votes

Can one prove that toric varieties are Cohen-Macaulay by finding a regular sequence?

In my paper arXiv:math/9802052 I give an argument in the graded case. The general case can be approached similarly (as sketched in the paper). The sequence in question is made from log-derivatives of …
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