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Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.
4
votes
how do we prove that a sum of two periods is still a period?
For a given $n \in \mathbb{N}$, consider the algebraic function:
$$ f_n : \mathbb{R}^{n+2} \rightarrow \mathbb{C} $$
$$ f_n(x,y,z_1,\dots,z_n) = x + iy $$
We're going to define a 'simple period' to be …
23
votes
Accepted
A cubic and six conics problem
Recall the quartic version of the Cayley-Bacharach theorem:
Theorem: Consider two quartics in general position, which intersect in $16$ points (by Bezout's theorem). Then if a third quartic passes th …
16
votes
2
answers
1k
views
Algebraic surface of constant width?
Does there exist an irreducible polynomial $f \in \mathbb{R}[x, y, z]$ such that:
$$ V := \{ (x, y, z) \in \mathbb{R}^3 : f(x, y, z) \leq 0 \} $$
is a solid of constant width with a finite symmetry gr …
18
votes
2
answers
697
views
Can all unit-distance graphs have their vertices at algebraic integers?
A graph $G$ is described as a unit-distance graph if there exists a function $f:G \rightarrow \mathbb{C}$ such that for every edge $(u,v) \in E(G)$, we have $|f(u) - f(v)| = 1$.
Obviously, we can nec …
11
votes
Accepted
Expected number of lines meeting four given lines or "what is 1.72..."
The integrand is periodic modulo $\pi$ in each variable, so it suffices to integrate each variable over $[0, \pi]$ and replace the constant factor by $2^{-7}$.
If we were to apply a change of variabl …
4
votes
Unusual symmetries of the Cayley-Menger determinant for the volume of tetrahedra
The following comment in the question intrigued me:
In fact, it's possible to show that the linear symmetries of
$\mathbb{R}^6$ that preserve the Cayley-Menger determinant form the
Weyl group $D_6$, …
31
votes
Is there any theory why (for Bitcoin) the discrete logarithm problem is so hard to solve?
Yes. I'll talk about why elliptic curve discrete log is harder than ordinary discrete log.
Suppose we have $g, h$ and want to find $n$ such that $g^n = h$.
The usual methods for solving the discrete …