Questions tagged [smoothness]
The smoothness tag has no usage guidance.
142
questions
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$C^{k, \alpha}$ gradients $\implies C^{k + 1, \alpha}$ level sets
Referring to the statement in the parentheses below (Regularity Theory for Elliptic PDE, Xavier Ros-Oton, p. 172)
-- the authors go forward to make a bootstrapping argument that uses this result:
...
- 235
1
vote
1
answer
42
views
From relative convexity to modulus of continuity estimates for the dual gradient mapping
Let $F: \mathbf{R}^d \to \mathbf{R}$ be a convex function, let $m > 0$, and define $Q_m: \mathbf{R}^d \to \mathbf{R}$ to be the mapping $x \mapsto \frac{m}{2} \| x \|_2^2$. One says that $F$ is $m$-...
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0
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Admissible representations of an $\ell$-group are a (neutral) Tannakian category?
Let $G$ be an $\ell$-group in the sense of Bernstein/Zelevinsky (sometimes also called td-group), i.e. $G$ is a
Hausdorff
locally compact
totally disconnected
topological group. Prominent examples ...
- 678
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0
answers
35
views
Approximation of function that has Lipschitz-continuous $n$-th derivative
Good afternoon. I'm trying to find in literature the solution for such a problem: for given function with $L_p$-Lipschitz continuous $p$-th derivative I need to find function $f_\varepsilon$ with $L_n$...
3
votes
2
answers
313
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The real dimension of any real algebraic set equals the complex dimension of its complexification
I want to prove the following statement. Please help!
Given any semialgebraic set $A$, consider its real Zariski closure $V_{\mathbb{R}}$ (which always has the same real dimension of $A$). Now ...
- 33
1
vote
0
answers
151
views
Can we check smoothness of a morphism after base change to the algebraic closure?
I know that smoothness is fppf local on the base, but this is not enough because taking algebraic closures is not finitely presented. The reason I'm asking this is because I want an easy/quick ...
- 409
1
vote
1
answer
216
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Is a Lipschitz continuous gradient equivalent to this condition?
I know if a function $f: \mathbb{R}^n \to \mathbb{R}$ is $L$-smooth, i.e. its gradient $\nabla f$ is $L$-Lipschitz continuous, then it satisfies the following inequality for any $x, x_0 \in \mathbb{R}^...
- 143
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votes
1
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347
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Are algebras of smooth functions formally smooth?
Let $M$ be a manifold. Then is the ring of smooth functions $C^\infty(M,\mathbb{R})$ formally smooth over $\mathbb{R}$?
If it helps, feel free to assume that $M$ is compact.
(This is not a joke ...
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2
votes
1
answer
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Smooth extension of functions at corners
Let $\mathbb{B}_1(0)\subseteq\mathbb{R}^n$ be the ball of radius $1$ in the Euclidean space, $n>1$. Suppose we have a cylinder $C=[0,1]\times \mathbb{B}_1(0)$ and suppose we are given smooth ...
- 23
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134
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Why does Deligne's construction of the Galois representation attached to the new cuspidal forms require that the Kuga-Sato manifold be regular?
The origin of this question is related to the construction of Galois representations of Deligne attached to $f$ a new cuspidal form (of weight $k\geq 2$). To do this, we consider the fiber product $k$-...
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Interpretation of $p$-adic 'smoothness'
real case: In the very first course of Calculus, one learns that a real function $f \colon \mathbb{R} \to \mathbb{R}$ is called smooth, if it is differentiable as many times as one pleases. So the ...
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votes
1
answer
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Is the Borel lemma projection a smooth principal bundle?
Consider the Fréchet spaces $C^\infty(\mathbb{R},\mathbb{R})$ and $\mathbb{R}^\infty$, and the continuous linear map
$$
J\colon C^\infty(\mathbb{R},\mathbb{R}) \to \mathbb{R}^\infty
$$
returning the ...
- 32.9k
3
votes
1
answer
291
views
Smooth approximation of the $\max\{0,x\}$ function with controlled derivatives
Motivation/Hand-Wavy Question:
In this post, it was asked what the best local approximation of $f(x):=\max\{0,x\}$ is by a polynomial of a given degree; with the answer provided by Chebyshev's ...
- 4,961
5
votes
1
answer
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A regular, geometrically reduced but non-smooth curve
Can anyone give an example of a projective, regular, geometrically reduced but non-smooth curve ?
Of course, the base field should be imperfect.
In Exercise 4.3.22 of Qing Liu's book Algebraic ...
- 600
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0
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36
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$H^1 \cap C^0$ boundary, smooth $H^1$ extension
Assume we have a $u \in H^1(\Omega; \mathbb{R}^n) \cap C^0$ where $\Omega$ is a bounded open Set with smooth boundary. Also $u\vert_{\partial \Omega} \in H^1(\partial \Omega; \mathbb{R}^n) \cap C^0$. ...
- 11
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0
answers
89
views
Smoothness of Radon transform
Let $f:\mathbb R^n \to \mathbb R$ be density function (i.e nonnegative function which integrates to $1$), and consider its Radon transform $R[f]$ defined by
$$
R[f](w,b) := \int_{\mathbb R^n}\delta(x^\...
- 6,466
1
vote
0
answers
128
views
Two definitions for smoothness
I'm currently reading Sarah Witherspoon's book on Hochschild Cohomology. At the beginning of the fourth chapter it is given the following definition:
Definition 1. If $k$ is a field and $A$ is a $k$-...
- 173
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votes
1
answer
151
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Smoothness of ruled surface (asymptotic) parameterisations
A ruled surface $S$ shall be defined as surface consisting of straight line segments. It is commonly known (cf. [BER, p.362] or [STR, p.93] - bibliography at the end) that a ruled surface allows for a ...
3
votes
0
answers
117
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Covering number $C^k$-balls in $C(\mathbb{R}^n)$
Fix a positive integer $n$ and and an non-negative integer $k$. The Arzela-Ascoli theorem guarantees that for a given positive integer $k$ and a given $L>0$ the set
$$
Ball_{C^{k,1}([0,1]^n)}(0,L)
...
- 4,961
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2
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293
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Smoothing a map $f:X\to \mathbb{R}$ while fixing it over a closed $C\subset X$
$\newcommand{\R}{\mathbb{R}}$I have a map $f\in C^0(X,\mathbb{R})$, where $X$ is a compact and Hausdorff topological space, which is a manifold outside of a compact subset $K\subset X$.
I would like ...
- 2,493
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89
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Can a smooth function always fit between two non-smooth functions? [closed]
Suppose I have two continuous functions, $f$ and $g$, with $f(x)<g(x)$ for all $x$ in some closed domain. Is it always possible to find a (piecewise, if needed) smooth function $h$ such that $f(x)&...
- 349
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1
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On the Lipschitz continuity of $x \mapsto \arg\min_{c \in C}d(x,c)$ w.r.t Hausdorff distance
Let $C$ be a (nonempty) compact subset of euclidean $\mathbb R^n$, and consider the set-valued map $p_C:\mathbb R^n \to 2^C$ defined by
$$
p_C(x) = \{c \in C \mid \|x-c\| = \mbox{dist}(x,C)\},
$$
...
- 6,466
7
votes
2
answers
560
views
Is $(x^2y,xy^2)$ log smooth?
Consider the map
$$f:\mathbb C^2\to\mathbb C^2$$
$$(x,y)\mapsto(x^2y,xy^2)$$
We can view $f$ as induced by the map of monoids $g:\mathbb Z^2_{\geq 0}\to\mathbb Z^2_{\geq 0}$ given by the matrix $(\...
- 18.1k
6
votes
2
answers
813
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Zero points of a smooth function on $\mathbb{R}$
Assume $f(x)$ is a smooth function on $\mathbb{R}$ and $f$ does not vanish on any interval. In other words, $f$ can have zero points but we cannot find any interval $(a, b)$ such that $f(x)=0$ for all ...
- 903
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0
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84
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Functions with smooth projections on finite-dimensional subspaces
Let $E,F$ be Banach spaces and $F$ be finite-dimensional and $E$ be strictly convex. Let $f\in C(F,E)$ have the property that:
$$
\text{For every finite-dimensional subspace $E'\subseteq E$ we have } ...
- 4,961
1
vote
0
answers
236
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A characterization of the integral
Let $I(f)$ be an endomorphism of the smooth functions with zero value in zero such that:
$$\ln[1+I(f)]=I\left(\frac{f}{1+I(f)}\right).
$$
Then, does it exist $g$ smooth such that:
$$I(f)(x)=\int_0^x f(...
- 377
2
votes
1
answer
89
views
The notion of smoothness in the local situation
I am reading Bump's book on Automorphic forms and Representations and I am able to draw a lot of parallels between the theory of $GL(2, \mathbb{R})$ which is the infinite place and the theory of $GL(2,...
- 497
6
votes
2
answers
275
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A smooth function such that the second derivative of its absolute value is a distribution of positive order
Let $f\in C^\infty(\mathbb R;\mathbb R)$ and let us define $g(x)=\vert f(x)\vert$. It is easy to verify that $g$ is locally Lipschitz-continuous function, but I would like to find an example of a ...
- 14.6k
5
votes
4
answers
544
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Relative version of Hilbert syzygy theorem
I presume that answers to the following questions are likely to exist in the literature; so this question is mostly a reference request (but failing that, I would be certainly interested in learning a ...
- 14.9k
6
votes
1
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Is every variety an image of a smooth variety?
Let $X$ be a finite type scheme over a field $k$.
Is it true that there exists a surjective morphism $f : Y \rightarrow X$, where $Y$ is smooth over $k$?
In other words, is every such scheme a ...
- 455
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votes
0
answers
61
views
(Linear combinations) Algebra of translated radial test functions [Radial basis functions]
Let $n \in \mathbb{N}$, $\Omega \subseteq \mathbb{R}^n$. Function $f \colon \Omega \to \mathbb{R}^n$ shall be called translated radial function if there exists $x \in \mathbb{R}^n$ and $g \colon [0, \...
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0
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133
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A question regarding base change of a smooth algebra via completion
Let $(R,m)$ be an excellent Noetherian local ring. Let $S$ be a smooth (i.e. $R \rightarrow S$ is flat and has geometrically regular fibers) Noetherian $R$-algebra. Let $T$ be the $m S$-adic ...
- 1,670
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0
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211
views
Transversal intersection with linear subspaces
Let us work over an algebraically closed field $K$. If $X\subset \mathbb{P}^n$ is a closed subset of dimension $r$, then there should exist a linear subspace $L\subset \mathbb{P}^n$ of dimension $n-r$ ...
- 7,522
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votes
0
answers
200
views
Effective bounds for a Bertini-type result
Suppose $X$ is a projective subvariety of $\mathbb{P}^n$ of codimension $r$ over $\mathbb{C}$, defined set-theoretically by $r$ homogeneous polynomials $P_1,\dots,P_r$ of degree at most $d$. By ...
- 527
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0
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etale locally infinitesimal lifting property
For a morphism $X\rightarrow Y$ of qcqs schemes, one has the usual notion of formal smoothness which says that for a pair $(R,I)$ with $I^2=0$, if there is a point $y\in Y(R)$ such that $y_{\vert R/I}$...
- 3,314
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2
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557
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smooth functions on closed intervals with values in infinite-dimensional spaces
There are three ways to define when a ($\mathbb{R}$-valued) function on a closed interval is smooth:
$f$ can be extended to a smooth function on $(a - \epsilon, b + \epsilon)$ for some $\epsilon > ...
- 1,091
1
vote
0
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147
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On smoothness and roughness of a number related to triangular numbers
Define $\triangle_n$ to be the $n$th triangular number.
Define $$M_n=(2\triangle_n-1)2\triangle_n(2\triangle_n+1)=2\triangle_n(4\triangle_n^2-1).$$
Define $(\ell,k)$-smough numbers to be numbers that ...
- 1,776
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votes
1
answer
143
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Critical Smoothness on Besov Spaces $B^s_{p}$: how does it evolved with $p$?
We denote by $B_{p}^s(\mathbb{T}) := B_{p,p}^s(\mathbb{T})$ the Besov space over the circle $\mathbb{T}$ with parameters $p=q \in (0, \infty]$ and smoothness $s \in \mathbb{R}$.
For $p>0$ fixed and ...
- 2,142
6
votes
1
answer
401
views
A smooth function $\mathbb{R}\to\mathbb{R}$ agrees with an analytic function on a bounded infinite set
Fix a smooth function $f:\mathbb{R}\to\mathbb{R}$. Do there exist real numbers $a<b$, an infinite set $S\subset (a, b)$ and an analytic function $g$ defined on $(a-\epsilon, b+\epsilon)$ for some $\...
user158636
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0
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How smooth can this be?
If $a$ is an even integer then how smooth can $a^2-1$ be?
Approximately how many integers in $a\in[0,t]$ are there such that $a^2-1$ is $k$-smooth?
- 1,776
6
votes
1
answer
263
views
On Glaeser's result for the square-root of a smooth non-negative function
One of the results due to Georges Glaeser is the following: there exists a non-negative $C^\infty$ function $f$ on the real line, flat at its zeroes, such that $\sqrt{f}$ is not $C^2$. On the other ...
- 14.6k
2
votes
1
answer
230
views
On the proof of "Mapping space is a Chen space"
According to the page 5 in the paper Convenient Categories of Smooth Spaces https://arxiv.org/pdf/0807.1704.pdf by Baez and Hoffnung, Chen space is defined as follows:
(Note:I used different ...
- 1,185
0
votes
0
answers
191
views
Understanding Krantz's proof of Hefer's lemma in $\mathbb{C}^2$
Note: I initially phrased the question in a different way, and it did not receive much attention. In the hope to make it more interesting, I have included a (long) introduction to contextualize and ...
- 11
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votes
0
answers
145
views
When is the quotient of a manifold by a discrete group of diffeomorphisms a diffeological covering space?
I was reading An Introduction to Diffeology by Patrick Iglesias-Zemmour and he defines a diffeological covering space as a diffeological fiber bundle with discrete fiber.
My question: Consider a ...
- 374
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votes
2
answers
582
views
Is the square root of a monotonic function whose all derivatives vanish smooth?
Let $g:[0,\infty] \to [0,\infty]$ be a smooth strictly increasing function satisfying $g(0)=0$ and $g^{(k)}(0)=0$ for every natural $k$.
Is $\sqrt g$ is infinitely (right) differentiable at $x=0$?
...
- 6,581
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votes
2
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328
views
On Glaeser's Theorem for non-smooth functions
Glaeser's Theorem says that a $C^\infty$ function $F$ on $\mathbb R^n$ which is invariant under permutation of the variables is a smooth function of the symmetric polynomials of $(x_1, \dots, x_n)$.
...
- 14.6k
2
votes
0
answers
167
views
Morphism between jet spaces smooth
In this article "Introduction to Jet Schemes and Arc Spaces" S. Ishii introduces the spaces of $m$-jets:
Let $X$ be a variety over algebraically closed field $k$. The space $X_m$ of $m$-jets ...
- 5,274
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votes
1
answer
236
views
Smoothness of the radius of convergence
Let $(x\mapsto a_n(x))_n$ be a sequence of smooth functions defined on some fixed interval $I$. Consider the power series $\sum_{n\geq 0}a_n(x)t^n$ and denote by $R(x)$ its radius of convergence. Does ...
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votes
0
answers
195
views
Lifting a Frobenius endomorphism under an étale morphism
Let $X$ be a smooth affine scheme over $\mathbb{Z}/{p^2}$ that is a complete intersection, say $X$ is the spectrum of $\mathbb{Z}/{p^2}[x_1,...x_n]/(f_1, ... f_r)$, where $n-r$ is the dimension of $X$....
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0
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139
views
Definition of Morphisms of algebraic stacks smooth of relative dimension n
There is a notion of smooth morphism of algebraic stacks e.g. Tag 075U and a notion of
relative dimension of a locally of finite type morphism $T\to \mathcal{X}$ from an algebraic space into an ...
- 91