# Questions tagged [semialgebraic-geometry]

Semialgebraic geometry is the study of inequalities expressible in algebraic functions in one or several variables, usually over the real numbers or some field with similar properties.

17 questions
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### The set of polytopes with given $f$-vector

Let $f=(f_0,\ldots f_n)$ be a vector in $\Bbb N^{n+1}$. Let $X$ be the set of all (ordered) $f_0$-tuples in $\Bbb R^n$ whose convex hull has $f$ as its $f$-vector. Assume that $X$ is non-empty. Is ...
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### Solutions to a system of homogeneous equations (inequalities)

Let $f_1,\ldots,f_r \in \mathbb{R}[x_1,\ldots,x_n]$ be $r$ homogeneous polynomials of the same odd degree $d$, where $d \in \{3,5,7,\ldots\}$. For which values of $r,n,d$ there exists a real ...
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### First order decidability of limit of gradient flow?

Let $f: \mathbb{R}^n\to\mathbb{R}$ be a polynomial function, and let $p$ be a critical point. Consider the ascending manifold $A_p$ consisting of all points whose limit under the gradient flow of $f$ ...
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### Polynomials that preserve nonnegativity

A polynomial $p \in \mathbb{R}[x_1,\dots,x_n]$ is said to be positive on a subset $S$ of $\mathbb{R}^n$ if $p(x) > 0$ for every $x \in S$. The polynomial $p$ is called nonnegative if $p(x) \ge 0$ ...
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### Is a spectrahedron's boundary almost always “smooth”?

A spectrahedron is a convex set defined by a linear matrix inequality (LMI). Is the boundary of such a set almost always smooth? By "smooth" I mean that it admits a tangent hyperplane at any point ...
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### Semialgebraic sets containing irrational power functions

Let $\alpha$ be an irrational number, and consider the set $A=\{(x,x^\alpha),x\ge 0\}\subseteq \mathbb{R}^2$, which is the graph of the function $f(x)=x^\alpha$. I'm trying to prove/disprove the ...
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I have an engineering problem that may be solved using semidefinite programming. I would like to know whether a given set is convex. Let $m \in \mathbb{R}^+$ be a positive real scalar, $l \in \mathbb{... 0answers 102 views ### Classification of$n$-dimensional Nash-submanifolds of$\mathbb{R}^n$Let$M,N\subset \mathbb{R}^n$be two open semi-algebraic subsets, and assume that$M$and$N$are$C^\infty$diffeomorphic, i.e. isomorphic as smooth submanifolds of$\mathbb{R}^n$. Does this imply ... 1answer 203 views ### Do convex closed semialgebraic hyperplane cross-sections imply semi-algebraicity? Let$S\subset\mathbb{R}^n$, with$n\geq 3$, such that for any hyperplane$L$one has$L\cap S$closed, semialgebraic, and convex. Is it true that$S$itself is semialgebraic? A colleague explained to ... 0answers 187 views ### Where do I read about semi-algebraic/analytic sets? What's a good introduction to semi-algebraic/semi-analytic sets? I'm coming from algebraic geometry, so ideally I'd prefer material that has a similar point of view and highlights analogies. I've ... 0answers 178 views ### Moduli in semialgebraic geometry Is there a moduli space in semialgebraic geometry analogous to the Hilbert scheme in algebraic geometry? The sort of thing I am imagining is an object in a category of semischemes: Ordinary schemes ... 2answers 252 views ### Convexity of a certain sublevel set Consider the polynomial of degree$4$in variable$r$$$r^4 + (x^2 + y^2)\ r^2 - 2 x y\ r + x^2 y^2$$ The discriminant of this polynomial in$r$is the following expression (obtained using ... 0answers 96 views ### Cross sections of semialgebric sets Let$n$be bigger than two, and let$A$be a subset of the$n$-dimensional Euclidean space. Suppose that the intersection of$A$with any$(n-1)$-dimensional affine hyperplane is semialgebraic. ... 1answer 167 views ### Ultrafilters on the set of semialgebraic subsets of R^2 Let$R$be a real closed field and$f: R \to R$a map. Then let$\textrm{F}(f)$be the set of semialgebraic subsets of$R^2$, which contain$(t,f(t))$for all$0< t< \epsilon$for some$\epsilon ...
Let $S$ be a (basic) semialgebraic set, for which we know the constraints (i.e. we know the polynomial inequalities and equalities which define the set). Is it necessarily the case that the cone \$C = \...