Most active questions
781 questions from the last 30 days
8
votes
1
answer
305
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Identity?: $\frac{\varphi(2^n-1)}{n}=\frac{2^{\varphi(2^n-1)}-1 \bmod (2^n-1)^2}{2^n-1}$
The computer found this.
Let $n$ be a positive integer.
Up to $n=200$ we have:
$$\frac{\varphi(2^n-1)}{n}=\frac{2^{\varphi(2^n-1)}-1 \bmod (2^n-1)^2}{2^n-1}. \tag{1}\label{483144_1}$$
Q1 Is \eqref{...
- 25.4k
5
votes
1
answer
369
views
Groups with no proper non-trivial fully invariant subgroup
Let $G$ be a finite group. A subgroup $H$ of $G$ is said to be characteristic if $\phi(H)\subseteq H$, $\forall \phi \in \operatorname{Aut}(G)$ and fully invariant if $\phi(H)\subseteq H$, $\forall \...
- 361
11
votes
1
answer
478
views
Closed formula for the factorial over reals
How to prove that there is no formula for $n!$ that does only use the binary operations $+,-,*,/$ on real numbers, powers of real numbers, and fixed real numbers?
Similar question have been asked ...
- 19.1k
4
votes
1
answer
550
views
Question on Lorentzian geometry
I apologize in advance if this is a too basic question.
Let $(M,g)$ be a Lorentzian manifold with signature convention $(-,+,\dots,+)$. Now, lets suppose $X\in\Gamma(TM)$ defines a global time-...
- 1,171
3
votes
3
answers
492
views
In hyperreal field, can ln(ε) and ln(ω) be expressed as infinite sums?
In the hyperreal field, we can use Taylor series to express e^(ε) and e^(ω) as:
e^(ε) = 1 + ε + (ε^2)/2! + ...
e^(ω) = 1 + ω + (ω^2)/2! + ...
Is it similarly possible to express ln(ε) and ln(ω) as ...
- 55
5
votes
3
answers
368
views
Asymptotics for minimum of a sequence of random variables
This is a question that I'm sure has been investigated before, but I have found no good search terms for.
Let $X_i$ be a sequence of random variables, independent and uniformly distributed in $[0,1]$. ...
- 28.2k
6
votes
1
answer
822
views
Are periodic functions such as sine and cosine defined on surreal numbers?
Surely, one can compose a power series for them, and any partial sum of those series would be defined, But are they defined in the limit?
I mean, what is $\cos \omega$, for instance?
Does the ...
- 10.1k
7
votes
1
answer
289
views
Group cohomology valued in a bimodule
The usual setup for group cohomology of a group $G$ is as follows. One takes a $G$-module $M$, and considers the space of all maps
$$\ell : G \times \cdots \times G \longrightarrow M $$
together with ...
- 9,853
8
votes
1
answer
401
views
Reduction of structure group and classifying spaces
Let $H, G$ be topological groups and $\phi : H \to G$ a group homomorphism. Let $M$ be a paracompact topological space.
For any principal $G$-bundle $P \to M$, a reduction (or sometimes 'lift') of its ...
- 113
7
votes
2
answers
158
views
On the continuity a function given by evaluating compact subsets of smooth functions
Let $M$ be a compact connected smooth manifold. Write $C^{\infty}(M)$ for the Frechet space of the smooth real-valued functions on $M$ equipped with the usual $C^{\infty}$-topology.
Given a compact ...
- 577
4
votes
2
answers
351
views
$K_2$ over finite fields and polynomials over finite fields
I am interested in presentations of the group $SL_n(\mathbb{F}_q)$ (and eventually $SL_n(\mathbb{F}_q[t])$).
The standard "Chevalley" presentation of $SL_n(R)$ for a ring $R$ has generators $...
- 173
6
votes
2
answers
510
views
Is every automorphism of a cone diagonalisable?
Let $V$ be a real, finite-dimensional vector space, and let $C\subset V$ be a closed convex cone, with nonempty interior, and such that $C\cap (-C)=\{0\} $. Let $u\in\operatorname{GL}(V) $ such that $...
- 38k
6
votes
1
answer
498
views
NBG, ZFC+I, and Global Choice
In Shulman's 2008 paper 'Set Theory for Category Theory', he includes amongst the axioms of $\sf NBG$ the axiom of limitation of size. Being well known to imply the axiom of global choice, it seems to ...
- 63
3
votes
1
answer
406
views
Moments of a random variable related to uniform distribution on sphere
Let $u$ be taken uniformly from the unit sphere $\mathbb S^{n-1}$ and $D$ be a diagonal matrix. I'd like to find a general formula for
$$
\mathbb E[(u^\top D u)^m]
$$
for $m=1,2,3, \dots$, in terms of ...
- 1,608
6
votes
1
answer
988
views
How many colors do we need?
How many different colors do we need so that the set of all possible colorings of $\mathbb{R}^3$ is greater than the powerset of $\mathbb{R}$.
Countably many doesn't seem to be enough and even $|\...
- 131
4
votes
1
answer
284
views
Do the two orientations on an orientable manifold $M$ uniquely witness lifts of $\tau_M: M \to B\text{O($n$)}$ to $B\text{SO($n$)}$?
For an orientable $n$-manifold $M$ and its (orthonormal) frame bundle classifying map $\tau_M : M \to BO(n)$, we have a lift diagram of the following sort:
There are two orientations on $M$. Is it ...
- 113
3
votes
2
answers
344
views
Cohomology version of Moore space
I asked this question on MSE a few days back but could not get any helpful response. So I am rewriting the post.
It is known to me that given a simply connected finite dimensional (which is also level-...
- 1,177
5
votes
1
answer
433
views
Number of roots of a quadratic form over GF(2)
If $Q(x) = x^T A x$ with $x \in GF(2)^n$ and $A \in GF(2)^{n \times n}$, is there a way to find how many roots $Q(x)$ has based on any properties of $A$ (e.g., rank, etc.)?
- 63
9
votes
1
answer
402
views
Conceptual understanding of the Néron–Severi group
I'm trying to understand the importance of the Néron–Severi group $\operatorname{NS}(X)$ when $X$ is, say a complex manifold. My background is in the analytic side so I'm much more familiar with line ...
- 137
11
votes
1
answer
375
views
Reference request: The non-productivity of Lindenbaum numbers
For a set $X$, the Lindenbaum number of $X$, $\aleph^*(X)$, is the least non-zero ordinal $\alpha$ such that there is no surjection $X\to\alpha$. It seems to be well-known that for infinite sets $X$ ...
- 2,206
4
votes
2
answers
207
views
Normalizers of the principal congruence subgroups in $\mathrm{GL}(n,\mathbf Q)$
A question quite similar to this question. Let $n \geqslant 3$ and $m \geqslant 2$ be natural numbers and suppose that a matrix $A \in \mathrm{GL}(n,\mathbf Q)$ normalizes the principal congruence ...
- 43
8
votes
2
answers
360
views
Is the unbounded derived $\infty$-category of a general abelian category stable?
Let $A$ be any abelian category. Consider the stable $\infty$-category $N_{dg}(Ch(A))$ defined as the differential graded nerve of the category of chain complexes $Ch(A)$ in $A$. We can define an $\...
- 115
7
votes
1
answer
833
views
Why are some heuristics successful?
Mathematicians sometimes use heuristics to form expectations about what might be true or false. For examples, see Matthew Emerton's answer to Why should I believe the Mordell Conjecture?, this blog ...
11
votes
2
answers
321
views
Cohomology of foliations and closed forms along the leaves
Let $M$ be a manifold equipped with a codimension one, transversely orientable, regular foliation $F \subset M$. Let $\alpha\in \Omega^k(M)$ be a differential form on $M$ that is not closed on $M$ ...
- 2,818
3
votes
2
answers
366
views
Rational divisors on Calabi–Yau threefolds
Following the construction of [2], consider the full subcategory $\mathcal{D}_0 \subset D^\flat(\operatorname{Coh} \omega_{\mathbb{P}^1})$ consisting of complexes whose cohomology objects are ...
- 317
6
votes
2
answers
469
views
About Grothendieck and special cases
I was recently reminded of a quote about (not by!) Alexander Grothendieck that I had read many years ago, I think in the 1990s or 2000s.
The quote was about the way in which Grothendieck solved ...
- 887
5
votes
1
answer
240
views
Galois action on Borovoi's algebraic fundamental group
In Borovoi's paper Abelian Galois cohomology of reductive groups, the algebraic fundamental group of a connected reductive group $G$ over a field $K$ of characteristic zero is defined as
$$\pi_1(G, T):...
- 53
5
votes
1
answer
324
views
Non-negative coefficients polynomials
Let $n \in \mathbb N$ and $P,Q \in \mathbb R_+[x]$.
Is it true that $(x+1)^n\neq (x-2)^2 \times P(x)+(x-4)^2 \times Q(x)$ ?
I have asked, this question here (*), two weeks ago, but no answers.
(*) ...
- 4,074
5
votes
1
answer
735
views
Can the Pythagorean theorem be proved using imaginary numbers?
Can the Pythagorean theorem be proved using imaginary numbers? The proof must avoid circular reasoning, of course.
I asked essentially the same question at MSE, but did not receive a definitive answer,...
- 3,577
3
votes
2
answers
384
views
Is this true of the frame bundle $\operatorname{Fr}(M)$?
On an orientable (Riemannian) $n$-manifold $M$, with orthonormal frame bundle $\operatorname{Fr}(M)$, we have that the tangent bundle classifying map $\tau_M : M \to B{\operatorname O(n)}$ lifts to $B{...
- 113
5
votes
1
answer
325
views
An inequality that may be of isoperimetric nature
I am trying to prove the following inequality: let $f,g:S^1\to R$ (here $S^1$ is the unit circle parametrized by arc-length) be differentiable and have zero mean. Then
$$
4\pi \int f(t) g(t)\, dt \le \...
- 797
5
votes
3
answers
292
views
The max-clique chromatic number of a graph
Let $G = (V,E)$ be a graph. Every clique, that is, complete subgraph, is
contained in a maximal clique with respect to $\subseteq$ (this is
an easy consequence of Zorn's Lemma). Let $\newcommand{\MC}{\...
- 48.9k
9
votes
1
answer
335
views
An elementary proof of the equivalence of the Bol and Moufang identities
By a well-known result of Bol (1937) and Bruck (1946), for any loop the following two identities are equivalent:
B: $x(y(xz))=((xy)x)z$
M: $(xy)(zx)=(x(yz))x$.
A proof of the equivalence (B)$\...
- 42k
3
votes
1
answer
316
views
Which abelian varieties over a local field can be globalized?
As the title says, if $\mathcal{A}$ is an abelian variety over $\mathbb{Q}_p$, is there a criterion as to if I should expect there to exist $A$ over $\mathbb{Q}$ such that
$$\mathcal{A}\cong A\times_{\...
- 4,418
3
votes
1
answer
389
views
Do we have Pohozaev's identity on compact manifolds without boundary？
Recently I got to know about Pohozaev's identity, and I calculated several examples. The basic idea is multiplying $x \cdot \nabla u$ on both sides of the equation, but I noticed that all the ...
- 809
5
votes
1
answer
370
views
Are PA and Counting Theory synonymous\bi-interpretable?
The following question is whether $\sf PA$ is synonymous or even bi-interpretable with a theory about counting objects in finite sets.
Counting Theory:
$\textbf{Logic:}$ Bi-sorted first order logic ...
- 11.3k
14
votes
1
answer
504
views
Is there an 'unnatural' topological construction of an algebraically closed field of positive characteristic?
It's well known that while there is a natural topological construction of a nearly algebraically closed field of characteristic $0$, algebraically closed fields of positive characteristic seemingly ...
- 13.1k
7
votes
1
answer
297
views
Lower bound on dimension required to disconnect manifold?
This question seems quite classical, but I don't quite know what subarea of topology it falls into.
Suppose that removing the set $S$ disconnects the 2-torus $\mathbb{T}^2 = \mathbb{R}^2\diagup\mathbb{...
- 2,621
5
votes
1
answer
378
views
Convergence of random functions
Suppose I have a sequence of random continuous functions, $f^{n} : [0, t] \to \mathbb{R}$. Suppose there also exists a random continuous function, $f: [0, t] \to \mathbb{R}$, defined on the same ...
- 85
9
votes
2
answers
431
views
Hermite–Fourier expansion for the median
Let $n$ be an odd positive integer. Let $M : \mathbb{R}^n \to \mathbb{R}$ be the median function: $M(x_1,\dots,x_n)$ is the median of $x_1,\dots,x_n$. What can be said about the Hermite–Fourier ...
- 24.7k
8
votes
1
answer
318
views
Fibers of generic smooth maps between manifolds of equal dimension
I have heard that the following is a "well-known"
Claim. Let $M$ and $N$ be smooth manifolds with equal dimensions and $M$ compact. Then a generic smooth map $f\colon M\to N$ has finite ...
- 501
8
votes
1
answer
363
views
Eigenvalues of a certain combinatorially defined matrix
Let $A_n$ be the matrix whose rows and columns are indexed by pairs
$(i,j)$ with $1\leq i,j\leq n$ and $i\neq j$ (so $A$ is an
$n(n-1)\times n(n-1)$ matrix), whose $((i,j),(k,l))$-entry is 0 if
$i=k$ ...
- 50.8k
5
votes
1
answer
261
views
Diagonal analogue of symmetric functions
Let $n$ be a positive integer and consider the ring $R$ of power series over $\mathbb{Q}$ in commuting variables $x_1,y_1,x_2,y_2,...$. Let the symmetric group $\mathfrak{S}$ of permutations of the ...
- 1,500
5
votes
1
answer
184
views
Localizing spaces at stable homotopy equivalences
According to Bousfield's theory of localization, given any homology theory $E_*$ one can produce a reflective localization of the category of (pointed) CW complexes, in the sense that any such CW ...
- 213
2
votes
1
answer
466
views
About the number of critical points of a function
Suppose that $f$ is a totally monotone function on $(0,\infty)$, so that $(-1)^n f^{(n)}\ge0$ for all $n=0,1,\dots$, $f(0+)\in(0,\infty)$, and $f(t)\sim\frac{1}{t^{\frac{3}{2}}}$ as $t\to\infty$. Can ...
- 49
3
votes
3
answers
255
views
Continuum-distanced complete, ultrametric space
Our professor asked us to find a complete metric space where the intersection of nested closed balls can be empty.
The following space is such an example, and I would like to learn more on it (since ...
- 637
12
votes
1
answer
595
views
+200
Fundamental group of the complement of a codimension two submanifold
Let $M$ denote an arbitrary smooth, closed, connected, n-dimensional manifold for $n\geq 4$. For every such $M$, does there exist a closed (not necessarily connected!) codimension two submanifold $S \...
- 1,174
7
votes
1
answer
161
views
When is a non-linear first-order ODE equivalent to a linear second-order ODE?
The Riccati equation $y'(x)+y(x)^2=f(x)$ is non-linear, but can be transformed into the linear equation $-u''(x)+f(x)u(x)=0$ via $y(x)=\frac{u'(x)}{u(x)}$.
Is there a general statement known about ...
- 3,065
6
votes
1
answer
481
views
Few doubts about "A new elementary proof of the Prime Number Theorem" by Richter
I'm working on Richter's "A new elementary proof of the Prime Number Theorem" paper.
I have some doubt about the proof of proposition 3.1
Here's the reference to the paper: https://arxiv.org/...
- 125
9
votes
1
answer
459
views
A conjecture related to Frankl's conjecture
Let $\mathcal{F}\subseteq2^{[n]},\emptyset\in\mathcal{F}$ be an union-closed family of sets. For $S\in\mathcal{F}$, let $w(S)$ be the number of subsets of $S$ in $\mathcal{F}$. Does there always exist ...
- 1,462