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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 …
Adam P. Goucher's user avatar
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 …
Adam P. Goucher's user avatar
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 …
Adam P. Goucher's user avatar
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 …
Adam P. Goucher's user avatar
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 …
Adam P. Goucher's user avatar
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 $D_6$, …
Adam P. Goucher's user avatar
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 …
Adam P. Goucher's user avatar