# Recent MathOverflow Questions

### Convex function as series of continuous convex functions

Math Overflow Recent Questions - Tue, 04/10/2018 - 05:49

Assume K is compact, convex set in $\mathbb{R^{d}}$ and a function $f$ is convex, nonnegative, lower semicontinuous function on K. The question is : Can we find the sequence $f_{n}$ of continuous, nonnegative, pointwise increasing to $f$, convex functions such that $f_{n+1}-f_{n}$ is convex on K. For one dimensional case is true.

I will be glad to any suggestions.

### Family of large deviation principles

Math Overflow Recent Questions - Mon, 04/09/2018 - 10:28

The following question may be a bit imprecise in its formulation, I guess however the problem I have in mind is clear. Although to me it looks like a fairly standard question, I couldn't find any reference approaching it so far and hope someone here can help.

Assume that for every $\epsilon>0$, $\lbrace X^{\epsilon}_{n}\rbrace_{n}$ satisfies a LDP with rate function $I^{\epsilon}$. Also, suppose that for every $n\in\mathbb{N}$ we have tightness for $\lbrace X_{n}^{\epsilon}\rbrace_{\epsilon}$ and let $\lbrace X_{n}\rbrace_{n}$ be a family of limit points. Does convergence of the $I^{\epsilon}$ to some rate function $I$ in a reasonable sense (say $\Gamma$ or Mosco), already imply a LDP for $\lbrace X_{n}\rbrace_{n}$ with rate function $I$? What more is needed?
Remark: I'm here particularly interested in Schilder-type LDPs.

### The value of parameters in SIR model

Math Overflow Recent Questions - Mon, 04/09/2018 - 10:17

I am modelling the outbreak of Zika Virus in Brazil in 2016 using the SIR model.

It is quite difficult to collect data because the number of confirmed cases are largely underrated. So far I have found a graph published by the WHO with the number of suspected and confirmed cases in every epidemiological week of 2016 (EW 1= 81000 suspected cases). I set this as the number of infected individuals at $t=0$ (1st January 2016).

The recovered/removed class considers the number of people who have recovered (pass from the infectious class) and have permanent immunity in addition to those who have died as a result of the disease. For the number of infected people that have recovered I obtain $(1−0.083)·8100$. However, the number of deaths due to Zika are very low (the mortality rate is lower than 1%). The major consequence of the disease is that pregnant women pass it on to their babies and they are born with microcephaly, therefore the official death rate is 8.3%. The data I found about ZIKV deaths was that 8 people died from Zika in Brazil in 2016 (when the outbreak was at its highest peak) and in 2015, 76 microcephaly deaths related to ZIKV were reported in the country.

Is it reasonable to set the number of people recovered/removed at $t=0$ as $R=(1−0.083)·8100$ or should I add any deaths as well? Alternatively I could simply set it equal $R=0$ and assume that it is the beginning of the epidemic.

(For the total population size of Brazil I have taken figures of 2016)

### Can we compute the Tristram–Levine signatures of a knot in $S^3$ using Jacobian with Fox partial derivatives?

Math Overflow Recent Questions - Mon, 04/09/2018 - 10:12

My question is in the tittle:

Can we compute the Tristram–Levine signatures of a knot in $S^3$ using Jacobian with Fox partial derivatives?

If the answer is yes, is there a reference for this.

### Computing Mertens' function in time O(sqrt(x)) - in practice

Math Overflow Recent Questions - Mon, 04/09/2018 - 10:06

As far as I know, there is one way currently known to -- in principle -- compute the Mertens function $M(x) = \sum_{n\leq x} \mu(n)$ in time essentially $O\left(x^{1/2}\right)$, namely, a modification of the Lagarias-Odlyzko method.

Mertens' function in time $O(\sqrt x)$

Computing the Mertens function

The Lagarias-Odlyzko method's main variant is designed to compute $\pi(x)$ in time essentially $O(x^{1/2})$. In the last few years, it has been implemented and optimized:

https://arxiv.org/abs/1203.5712

https://arxiv.org/abs/1410.7008

Has the analogue for computing $M(x)$ ever been implemented? If not, why not? Are there significant ways in which this variant of Lagarias-Odlyzko would be much slower or harder to implement than the version for $\pi(x)$?

(One guess is that evaluating residues of $1/\zeta(s)$ may be hard. I know that this is a difficulty when one tries to obtain explicit forms of analytic results on $M(x)$, but I do not know whether the issue is at all relevant to Lagarias-Odlyzko and similar computational methods.)

### Does there exist a candidate for 'holomorphic' instanton Floer homology?

Math Overflow Recent Questions - Mon, 04/09/2018 - 09:34

The Euler characteristic of instanton Floer homology agrees with the Casson invariant. Thomas introduced the notion of holomorphic Casson invariant, defined using the holomorphic Chern-Simons functional. Is there any reasonable way a 'holomorphic' instanton Floer homology might exist?

### Different norms in decompositions of matrices

Math Overflow Recent Questions - Mon, 04/09/2018 - 09:22

It is known that the nuclear norm (trace norm) $\|A\|_*$ of a complex matrix $A$ is less than 1 if and only if $A$ can be written as a convex combination $$A = \sum_i c_i x_i y_i^*$$ for non-negative coefficients $c_i$ such that $\sum_i c_i = 1$ and some vectors $x_i, y_i$ whose Euclidean norm satisfies $\|x_i\| = \|y_i\| = 1$.

Now, say that $\|A\|_*\leq 1$, and additionally we know that A can be written as a convex combination $$A = \sum_i c'_i x'_i y'^{*}_i$$ where now the vectors $x'_i, y'_i$ satisfy $\|x'_i\|_\infty \leq \mu$ and $\|y_i\|_\infty \leq \mu$ for some $\mu < 1$.

Does this necessarily mean that A can be written as a convex combination $$A = \sum_i c''_i x''_i y''^{*}_i$$ with the vectors satisfying both $\|x''_i\| = \|y''_i\| = 1$ and $\|x''_i\|_\infty \leq \mu$, $\|y''_i\|_\infty \leq \mu$ ? Or perhaps $\|x''_i\|_\infty \leq \mu'$, $\|y''_i\|_\infty \leq \mu'$ for a different choice of $\mu'$?

I am trying to understand what exactly we can say about $A$ if we know that it admits the first two convex decompositions as mentioned above. Could we say more in cases when, for example, A is positive semidefinite?

Any ideas and thoughts about this problem would be appreciated. I have been stuck on this for a long time and not able to come up with any good way to approach it at all.

### My colleague and I have deduced some incredibly useful information based on a few widely-unknown formulae. Take a look! [on hold]

Math Overflow Recent Questions - Mon, 04/09/2018 - 09:20

These deductions are found primarily with the laws of pentallelograms and aerodynamics.

Work

### How to equate a text on left hand side to an inserted image on right hand side in latex? [on hold]

Math Overflow Recent Questions - Mon, 04/09/2018 - 09:18

I want the text in left hand side to be equated to an inserted image on left hand side as shown below:

Here, i wish the text in left hand side to be written in latex and equate it horizontally to an inserted image on right hand side. Also, how to insert text between images horizontally?

### Is the Milnor boundary map, a natural transformation?

Math Overflow Recent Questions - Mon, 04/09/2018 - 09:00

Consider the Milnor $K_n$-functors for discrete valuiation fields. For any discrete valuation field $F$ we can associate an abelian group $K_n(F)$ and the construction is given thanks a universal property involving Steinberg map. We also have a map called $r$-th boundary map, which is given by: $$\partial_n: K_n(F)\to K_{n-1}(\overline F)$$

We can see $\partial_n$ as a transformation between functors, and my question is the following one:

Is $\partial_n$ a natural tranformation?

Suppose that we have an embedding of discrete valuation fields $F\to L$ which gives $\overline F\to \overline L$; then is the following diagram commutative?

### Connection on a Principal bundle and transition functions, as in Hitchin's notes

Math Overflow Recent Questions - Mon, 04/09/2018 - 09:00

This is along the lines of this question on gerbes.

Gerbes are not just topological objects: we can do differential geometry with them too. We shall next describe what a connection on a gerbe is. To begin with, let’s look at a connection on a line bundle which is given by transition functions $g_{\alpha\beta}:U_\alpha\cap U_\beta\rightarrow S^1\subseteq \mathbb{C}^*$.

A connection on consists of $1$ forms $A_\alpha$ defined on $U_\alpha$ such that on a twofold intersection $U_\alpha\cap U_\beta$ we have $iA_\alpha-iA_\beta=g_{\alpha\beta}^{-1}dg_{\alpha\beta}$

I know what is a connection $1$ form but not as in above version. I am trying to relate what I know with what is given here.

Let $\pi:P\rightarrow M$ be a principal $G$ bundle with $\mathfrak{g}$ being the lie algebra of $G$.

Transition functions $g_{\alpha\beta}:U_\alpha\cap U_\beta\rightarrow G$ are as in this question

Definition : A connection form on $P$ is a $\mathfrak{g}$ valued $1$ form $\omega$ on $P$ such that

• $\omega(p)(A^*(p))=A$ for all $A\in \mathfrak{g}$ and $p\in P$
• $(\delta_g^*\omega)(p)(v)=Ad_{g^{-1}}(\omega(p)(v))$ for all $p\in P,g\in G$ and $v\in T_pP$.

Given a connection $1$ form $\omega$ on $P$ I am trying to associate a collection of $1$ forms $\{A_\alpha\}$ with some compatability conditions (here $A_\alpha$ is a $1$ form on $U_\alpha$).

Given local trivialization $\psi_\alpha$, we have a section of $\pi$ namely $\sigma_\alpha:U_\alpha\rightarrow P$ defined as $\sigma_\alpha(x)=\psi_\alpha^{-1}(x,e)$. Given local trivialization $\psi_\beta$, we have a section of $\pi$ namely $\sigma_\beta:U_\beta\rightarrow P$ defined as $\sigma_\beta(x)=\psi_\beta^{-1}(x,e)$.

Suppose $x\in U_\alpha\cap U_\beta$ then, we have $\sigma_\alpha(x)\in \pi^{-1}(x)$ and $\sigma_\beta(x)\in \pi^{-1}(x)$. Thus, there exists $g\in G$ (depending on $x$) such that $\sigma_\alpha(x)=\sigma_\beta(x)g$. Given $x\in U_\alpha\cap U_\beta$ there is an obvious choice for an element of $G$ namely $g_{\alpha\beta}(x)$. I could not prove (I am missing something obvious) but have seen that the $g$ that satisfy the condition as mentioned above is actually $g_{\alpha\beta}(x)$ i.e., we have $\sigma_\alpha(x)=\sigma_\beta(x)g_{\alpha\beta}(x)$ for all $x\in U_{\alpha}\cap U_\beta$ i.e., $\sigma_\alpha=\sigma_\beta g_{\alpha\beta}$.

Given a $1$ form $\omega$ on $P$ and we can pull back $\omega$ to $U_\alpha$ under $\sigma_\alpha$ to get $1$ form $\omega_\alpha=\sigma_\alpha^*\omega$ on $U_\alpha$ similarly we can pull back to $U_\beta$ to get $1$ form $\omega_\beta=\sigma_\beta^*\omega$ on $U_\beta$.

As $\sigma_\alpha$ and $\sigma_\beta$ are related by $\sigma_\alpha=\sigma_\beta g_{\alpha\beta}$, one can expect that $\omega_\alpha$ and $\omega_\beta$ are related some how. Given $g_{\alpha\beta}:U_{\alpha\beta}\rightarrow G$ we can produce a $1$ form on $U_\alpha\beta$ as pull back of $\theta$ on $G$ i.e., the canonical $1$ form on $G$ which is a left invariant $1$ form determined by $\theta(e)(A)=A$ for all $A\in \mathfrak{g}$. Let us denote pull back of $\theta$ to $U_\alpha\cap U_\beta$ by $\theta_{\alpha\beta}$. Then, I am expecting some compatibility relation between $1$ forms $\omega_\alpha,\omega_\beta$ and $\theta_{\alpha\beta}$ that should come from $\sigma_\alpha=\sigma_\beta g_{\alpha\beta}$.

Question : What could be reasonable relation between $\omega_\alpha,\omega_\beta$ and $\theta_{\alpha\beta}$ and how do I see that relation giving $iA_\alpha-iA_\beta=g_{\alpha\beta}^{-1}dg_{\alpha\beta}$ as a sepcial case.

Any suggestion on how to see this is welcome.

### Noncompact dual of $\mathrm{Spin}(2n)$ corresponding to $\mathfrak{so}^*(2n)$

Math Overflow Recent Questions - Mon, 04/09/2018 - 08:50

Let $U=\mathrm{Spin}(2n)$, which is a simply connected compact simple Lie group, and let $\mathfrak{u}_0=\mathfrak{so}(2n)$, the Lie algebra of $U$. If $\mathfrak{g}_0$ is a noncompact dual of $\mathfrak{u}_0$ in the complexified Lie algebra $\mathfrak{g}=\mathfrak{so}(2n,\mathbb{C})$, namely $\mathfrak{g}_0=\mathfrak{u}_0^\theta+\sqrt{-1}\mathfrak{u}_0^{-\theta}$ for some involutive automorphism $\theta$ of $\mathfrak{u}_0$, where $\mathfrak{u}_0^{\pm\theta}=\{X\in\mathfrak{u}_0\mid\theta(X)=\pm X\}$. Then there exists a noncompact closed subgroup $G$ of $G_\mathbb{C}=\mathrm{Spin}(2n,\mathbb{C})$. For example, if $\mathfrak{g}_0=\mathfrak{so}(m,2n-m)$, then $G=\mathrm{Spin}(m,2n-m)$.

QUESTION

What is $G$ when $\mathfrak{g}_0=\mathfrak{so}^*(2n)$?

I am not sure whether the question fits the level of MathOverFlow. I would like to say sorry if the question is too fundamental to be posted here.

### Singular Value Decomposition of product of a lower unitriangular and upper unitriangular matrices

Math Overflow Recent Questions - Mon, 04/09/2018 - 08:27

Let L be an $n\times n$ matrix with $(L)_{ij}=\mathbb 1(i\le j)$. Let the singular value decomposition of $LL'$ be $UDU'$. Is there an analytical form for $U$ and $D$, only as function of $n$?

### Number of collinear ways to fill a grid

Math Overflow Recent Questions - Mon, 04/09/2018 - 08:18

A way to fill a finite grid (one box after the other) is called collinear if every newly filled box (the first excepted) is vertically or horizontally collinear with a previously filled box. See the following example:

Let $g(m,n)$ be the number of collinear ways to fill a $m$-by-$n$ grid. Note that $g(m,n) = g(n,m)$.

Question: What is an explicit formula for $g(m,n)$?

Conjecture (user44191): $g(m,n)=m!n!(mn)!/(m+n-1)!$.

Definition (user44191): Let $g(m,n,i)$ be the number of collinear ways to fill $i$ boxes in a $m$-by-$n$ grid such that every row and every column contain at least one filled box.

Remark: $g(m,n) = g(m,n,mn)$.

Proposition (user44191): Here is a recursive formula for $g(m,n,i)$:

• $g(1,1,1) = 1$.
• If $m=0$ or $n=0$ or $i< \min(m,n)$, then $g(m,n,i) = 0$.
• $g(m,n,i+1)=(mn-i) g(m,n,i) + mn g(m-1,n,i) + mn g(m,n-1,i).$

Proof: The two first points are obvious. We consider the number of collinear ways to fill $i+1$ boxes in a $m$-by-$n$ grid such that every row and every column contain at least one filled box.
There are three cases, corresponding to the three components of the recursive formula:

1. The last filled box is not the only filled box in its row and not the only filled box in its column.
2. The last filled box is the only filled box in its row.
3. The last filled box is the only filled box in its column.

By the collinear assumption, 2. does not overlap 3. $\square$

One way to answer the question is to prove the conjecture using the above recursive formula.

We checked the conjecture for $1\le m \le n \le 5$, using the recursive formula (see below).

Remark: This question admits an extension to higher dimensional grids.
Remark: This question was inspired by that one.

Sage program

# %attach SAGE/grid.sage from sage.all import * import copy def grid(m,n,j): if [m,n,j]==[1,1,1]: return 1 elif j < min(m,n) or m==0 or n==0: return 0 else: i=j-1 return (m*n-i)*grid(m,n,i) + m*n*grid(m-1,n,i) + m*n*grid(m,n-1,i) def IsFormulaCorrect(m,n): return grid(m,n,m*n)==factorial(m)*factorial(n)*factorial(m*n)/factorial(m+n-1) def CheckFormula(M,N): for m in range(1,M+1): for n in range(M,N+1): if not IsFormulaCorrect(m,n): return False return True

Computation

sage: CheckFormula(5,5) True

### Information on structure (CI-magma with (non surjective)homorphism) of chemical transformations

Math Overflow Recent Questions - Mon, 04/09/2018 - 07:28

Thinking about the mathematical structure of chemical transformations, between all possible components (educts, products) it occurs to me, that this structure is a commutative-idempotent groupoid(=magma) (CI-groupoid). Call it $(\mathcal{X},\cdot)$.

Take reactants $a,b\in\mathcal{X}$ (one can imagine any kind of substance mixture) and react them. The set is closed under $\cdot$ by definition: $$a\cdot b \in\mathcal{X},$$ its commutative $$a\cdot b = b\cdot a ,$$ to say you react $a$ with $b$ is the same than to say you react $b$ with $a$ (though, "pouring one into the other" isn't, but this should be ignored here) and idempotent

$$a \cdot a = a.$$

I cannot pinpoint much more at the moment, associativity does not hold and inversion does not exist in general. So that should be a CI-groupoid, or CI-magma. Dropping the dots from now on, second dilution of $a$ in $b$ is $$(ab)b \ne a(bb) = ab.$$

$(\mathcal{X},\cdot)$ is now a kind of messy children chemistry where you end up mixing everything with everything and never separating anything.

Separation of components (carbon and oxygen gives CO$_2$ and CO) apparently is the identification of more structure:

$$ab=cd$$

Ultimately $\mathcal{X}$ is finitely generated, since ultimately there is only a finite number of molecules in the universe. Explicit generation from the finite number of elementary components (materials composed of only one element="elements"), however will afford inclusion of reaction conditions into the description.

Same reactants can yield different products under different reaction conditions. Reaction conditions can be described like applying an indexed map $\chi_i$ where in $i\in I$ the reaction conditions are encoded. The maps are apparently (non surjective) homomorphisms on $\mathcal{X}$ $$\forall a,b\in \mathcal{X}:\; \chi_i(ab)=\chi_i(a)\chi_i(b) \in \mathcal{X}.$$

Now my question, is there any developed theory/body of literature about such an algebraic structure? I have searched the internet for quite a while but only found sparse mentions of the term "CI-groupoid". Is there another maybe more common term which escaped me? Is it expected that any interesting comes out of such a categorisation (since there is so to say not really much structure)?

(I am not aware of this description in chemical research, but there is quite a lot going on in "chemical information theory" a field which is usually "below the radar" for mosts Chemists, and I do not expect that anyone here might be aware of developments in this specific field.)

### Does there exist concept of angles and sides in Non-archimedian field geometry?

Math Overflow Recent Questions - Mon, 04/09/2018 - 06:45

I am studying and trying to understand about Non-Euclidean Geometry for my future purpose.

An example of Non-archimedian field is the field of $\ Q-adic \ \ numbers \$ .

I have information about ultrametric space and the information that any triangle in an Non-archimedian field is Isoscale triangle. It is ok to me.

But right here I have a question.

Does there exist concept of angles and sides in Geometry over Non-archimedian field as in our ordinary Euclidean Geometry ?

Any help would be appreciated if you share something that can help me

### Does the axiom of choice follow from the statement "Every simple undirected graph is either connected, or its complement is connected"?

Math Overflow Recent Questions - Mon, 04/09/2018 - 06:23

Using the Well-Ordering Principle, which is equivalent to the Axiom of Choice, it can be proved that

(S): for every simple, undirected graph $G$, finite or infinite, either $G$ or its complement $\bar{G}$ is connected.

Does (S) imply (AC)?

### An inequality regarding projection

Math Overflow Recent Questions - Mon, 04/09/2018 - 05:30

Let $a, b \in \mathbb{R}^k$ be two normalized vectors such that $a^T b \ll 1$. Define matrix $C$ such that $[a, b, C]$ is full column rank, and let matrix $D$ be positive definite. Define projection matrix $P_A:=A^T (AA^T)^{−1}A$. Can we say the following?

$$\frac{a^T D^{-1/2}\left(I - P_{D^{-1/2}C}\right)D^{-1/2}b}{ \left\|\left(I - P_{D^{-1/2}C}\right)D^{-1/2}b\right\|_2 \left\|\left(I - P_{D^{-1/2}C}\right)D^{-1/2}a\right\|_2 }\ll 1$$

Also posted on MSE.

### Bound of the measure of the support of a set of divisors in a fixed linear system

Math Overflow Recent Questions - Mon, 04/09/2018 - 03:28

Let $(X,L)$ be a compact polarized complex manifold of dimension $n$. Let $\varphi$ be a smooth positive metric on $L$. Define $\omega=dd^c\varphi$. We shall use $MA(\varphi)=\omega^n$ as the measure on $X$. Then there is a natural $L^2$-inner product on $H^0(X,L)$. Now fix $s\in H^0(X,L)$ of norm $1$. Consider the following set $$A_\epsilon=\{s'\in H^0(X,L): \|s'\|=1, |(s,s')|<\epsilon\},$$ where $\epsilon>0$. My question is: how can we get an upper bound of $$MA(\varphi)\left(\bigcup_{s'\in A_{\epsilon}} Z_{s'}\right),$$ where $Z$ denotes the zero locus.

### A question on completely continuous operators

Math Overflow Recent Questions - Mon, 04/09/2018 - 03:12

An operator $T:X\rightarrow Y$ is said to be completely continuous if $T$ maps weakly convergent sequences to norm convergent sequences.

Let $Q: l_{1}\rightarrow l_{2}$ be any surjection and $J:l_{1}\rightarrow Y$ be an isomorphic embedding.

Question. Is there a completely continuous operator $S:Y\rightarrow l_{2}$ such that $Q=SJ$?

Thank you!