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# MathOverflow : Recent Questions and Answers (Page 44)

## A G-delta-sigma that is not F-sigma?

A subset of $mathbb{R}^n$ is$G_delta$ if it is the intersectionof countably many open sets$F_sigma$ if it is the union of countably many closed...

Asked on 08/20/2020 by Julián Aguirre

## How to explain to an engineer what algebraic geometry is?

This question is similar to this one in that I'm asking about how to introduce a mathematical research topic or activity to a non-mathematician: in this case algebraic geometry,...

## Conway's lesser-known results

John Horton Conway is known for many achievements:Life, the three sporadic groups in the "Conway constellation," surreal numbers, his "Look-and-Say" sequence analysis, the Conway-Schneeberger $15$-theorem, the...

Asked on 08/15/2020 by Joseph O'Rourke

## What is an example of a smooth variety over a finite field F_p which does not embed into a smooth scheme over Z_p?

Such an example of course could not be projective and would not itself lift to Z_p. The context is that one can compute p-adic cohomology of a variety X over...

Asked on 08/09/2020 by David Zureick-Brown

## On a geodesic mapping of a square

Let $X$ be a proper geodesic space which is uniquely geodesic. Let $phi:[0,1]times[0,1] to X$ be a function which satisfies the following: The maps $phi(0,cdot)$, $phi(cdot,0)$,...

Asked on 08/08/2020 by Logan Fox

## Hard to find book in number theory– Proceedings of conference in Budapest 1987

I'm looking for a copy of this book, which is hard to find: Number theory. Vol. I.Elementary and analytic. Papers from the conference held in Budapest, July 20–25, 1987....

## Meaning of L-reduction from Dominating set problem

We are working in a variation of Locating dominating sets. Recently, we realized that the reduction from dominating set to our problem in proving its NP-completeness turns out to be...

Asked on 08/07/2020 by Venugopal K

## Relating smooth concordance and homology cobordism via integral surgeries

Let $K_0$ and $K_1$ be knots in $S^3$. They are called smoothly concordant if there is a smoothly properly embedded cylinder $S^1 times [0,1]$ in...

Asked on 08/02/2020 by Maxim Ulyanov

## Quiver and relations for blocks of category $mathcal{O}$

In Vybornov - Perverse sheaves, Koszul IC-modules, and the quiver for the category $mathscr O$ an algorithm is presented to calculate quiver and relations for blocks...