# Tagged Questions

**0**

votes

**0**answers

77 views

### a question on sum of Gaussian binomial coefficients

I was trying to calculate something and at some point I get the following sum:
\begin{equation}
\sum_{t=0,t \text{ even}}^{s}{s+3n \brack s-t}\sum_{i = 0}^{t/2}q^{2i^2}{t/2+2n-i \brack t/2-i}{n ...

**10**

votes

**0**answers

318 views

### Erdos multiplication problem revisited

The well-known problem is acquiring a cardinality of the set of distinct numbers in the multiplication table n x m.
The very problem has been discussed in-depth and, as such, I require no further ...

**3**

votes

**3**answers

279 views

### How to find an integer set, s.t. the sums of at most 3 elements are all distinct?

How to find a set $A \subset \mathbb{N}$ such that any sum of at most three Elements $a_i \in A$ is different if at least one element in the sum is different.
Example with $|A|=3$: Out of the set $A ...

**15**

votes

**0**answers

311 views

### Spencer's “six standard deviations” theorem - better constants?

This question is about Joel Spencer's famous "six standard deviations" theorem. The theorem says that when
$$
L_i(x_1,\dots,x_n) = a_{i1} x_1 + \dots + a_{in} x_n, \quad 1 \leq i \leq n,
$$
are $n$ ...

**1**

vote

**0**answers

151 views

### Number of Orbits of symmetric group acting on $(\mathbb{Z}/n)^{l}$ [migrated]

I have encountered a problem that I suspect has been thoroughly studied but I have not been able to find references. Can anyone point me to a published reference dealing with this or a closely related ...

**11**

votes

**2**answers

358 views

### how to find cubic polynomial that an unknown subset of a set of integers satisfies

I have a set, $S$, of positive integers and I have reason to believe that some infinite subset of them may be parametrized by a cubic polynomial with integer coefficients evaluated at integer ...

**18**

votes

**3**answers

598 views

### Number of primitive $n$th roots with positive versus negative real parts

Does anyone know a reference to the following results, which I can prove, but I suspect may be known. Let $R(n)$ denote the number of primitive $n$th roots of unity with positive real part, and $L(n)$ ...

**8**

votes

**2**answers

310 views

### Sum and product estimate over integers, rationals, and reals

My question is the following: is finding a lower bound for $|A+A\cdot A|$ (as a function of $|A|$) where $A$ is any finite subset of the positive integers equivalent to finding the same lower bound ...

**11**

votes

**1**answer

559 views

### Colourings of $\mathbb Q\times \mathbb Q$ in three colours

Using two-adic valuation Monsky coloured $\mathbb Q\times \mathbb Q$ in red, blue, and green, so that on each line points of at most two colours are present.
Question. I would like to know if there ...

**4**

votes

**1**answer

203 views

### Circulant matrix with integer entries and determinant 1 or -1

CONJECTURE
Let $A= (c_0,c_1,\ldots,c_n)$ be a circulant matrix, i.e if $(c_0,c_1,\ldots,c_n)$ is the first column of $A$ then the $i$th column of $A$ is obtained by applying the permutation ...

**4**

votes

**3**answers

569 views

### Analogy between Integers and Permutations

I am reading Andrew Granville's Anatomy of Integers and Permutations where it is argued the factorization of a permutation into disjoint cycles is analogous to the factorization of a number into prime ...

**0**

votes

**0**answers

115 views

### Non existence (or existence) of a set that is equidistributed modulo $q$ for every $q$

I have been thinking about some set that is equidistributed modulo $q$,
uniformly in $q$ in some sense. I was starting to think this particular condition, which I describe below, is too strong and ...

**6**

votes

**5**answers

564 views

### What makes a set random?

There are many results in number theory, where the existence
of some $B \subseteq \mathbb{N}$ with certain properties is proved by
a probabilistic argument employing "random sets". One such example
...

**7**

votes

**0**answers

100 views

### Ordering subsets of the cyclic group to give distinct partial sums

Suppose that you are given a set $S$ of $k$ nonzero elements from $\mathbb{Z}_n$. Is it always possible to order the elements of $S$, say $a_1,a_2,\dots,a_k$ in such a way that the partial sums ...

**2**

votes

**1**answer

252 views

### Bound for a combinatorial sum

I was playing around with a problem and I obtained a certain
combinatorial sum. I was wondering if there was a way to simplify or bound it.
I have a real valued function $f$, which satisfies $|f(x)| ...

**4**

votes

**2**answers

337 views

### Can we sometimes define the parity of a set?

Suppose that ${n\choose k}, {n-1\choose k-1}, \ldots, {n-k+1\choose 1}$ are all even. (This happens for example if $k=2^\alpha-1$ and $n=2k$.) In this case, can we select ${n\choose k}/2$ sets of size ...

**4**

votes

**1**answer

268 views

### Existence of a certain subset of natural numbers equidistributed modulo $m$ for every $m$

I was talking to a friend and the following set $S$ came up.
Let $f$ be some real valued function tending to infinity.
Let $S$ be a subset of natural numbers such that $|S \cap [1,N]| = N^{\delta}+ ...

**1**

vote

**1**answer

235 views

### An infinite product: combinatorial interpretation

It is an undergraduate exercise to show that the generating function for the sequence of unrestricted integer partitions $p(n)$ is the celebrated infinite product
...

**3**

votes

**0**answers

97 views

### Number cubes with consecutive line sums

This is barely of research interest, but I'd classify it as a curiosity with connections to combinatorics.
The problem is to place integers in an $n \times n \times n$ array so that all $3n^2$ line ...

**35**

votes

**6**answers

2k views

### Placing numbers $1,2,\ldots,n^3$ in a cube so that numbers of any two adjacent unit subcube are coprime

This is a question first I asked in SE but since there was no suggestion or solution, I decide to put it here.
Consider an $n\times n \times n$ Cube containing $n^3$ unit cubes. Is it possible to ...

**2**

votes

**2**answers

809 views

### Number of 1 in binary representation of n

Let $1(n)$ be the number of digits $1$ in binary representation of number $n$.
For example, $13=1101_2$ so $1(13)=3\\$
Is there explicit form of $\,\,\sum{1(i)x^i} $?
I checked OEIS and didn't find ...

**9**

votes

**3**answers

726 views

### Could a perfect squared square be split into two perfect squared squares?

This is a geometric puzzle though it might conceivably
also define a special class of Pythagorean triples.
A perfect squared square PSS is a square (as a plane figure)
partitioned into smaller ...

**4**

votes

**1**answer

360 views

### Balog-Szemeredi-Gowers with dilates of sets

All sets are assumed to be finite subsets of the integers.
The additive energy of two sets $E(A,B)$ is defined as the number of solutions to $a+b=a'+b'$ with $a,a'\in A$ and $b,b'\in B$. The ...

**1**

vote

**1**answer

191 views

### Simplifying a sum in terms of divisor function Cauchy products

I'm trying to simplify this combinatorial looking sum:
$$\sum_{ax+by=n}_{(a,x,b,y)\in \mathbb{N^4}}\max{\{a,b\}}$$
In terms of possibly some scaled divisor functions plus a Cauchy product/convolution ...

**3**

votes

**0**answers

134 views

### What is the function like when its Mobius inversion is $\sum_{w|r, (w,t)=1}\mu(w)q^{r/w}$?

Everyone, I am now reading a paper named The Irreducible Factors of $(cx+d)x^{q^m}-(ax+b)$ over $GF(q)$, http://qjmath.oxfordjournals.org/content/14/1/61.extract. And Iâ€™m confused with one of its ...

**5**

votes

**1**answer

268 views

### Number of partitions whose blocks form arithmetic progressions

As is known, the set $\{1,\ldots,n\}$ has $2^n$ many subsets and $B_n$ (the $n$th Bell number) many partitions, where clearly $B_n<2^{2^n}$ and it is actually known that $B_n<n^n$ for large $n$. ...

**1**

vote

**0**answers

50 views

### A lower bound on the number of matrices whose image contains all multiples of $p^e$

Let $0\leq e<e^\prime$ be integers. Now suppose $N$ is the number of $n\times n$ matrices over the ring $R:=\mathbb{Z}/p^{e^\prime}\mathbb{Z}$ (where $p$ is prime) such that ...

**8**

votes

**0**answers

123 views

### Erdös-Fuchs Theorem for multivariate linear forms

Let $A$ be an infinite set of positive integers, and denote by $r(n)$ the number of solutions to the equation $a+a'=n$, with $a,\, a' \in A$.
It is not very difficult to show that if $r(n) > 0$ ...

**7**

votes

**2**answers

844 views

### Linear independence of the square roots over Q

Does there exist a real number $a$ such that the numbers $\sqrt{n^2 + a^2}$ (for all natural $n$) are linearly independent over the field of rational numbers? It is evident that $a$ cannot be ...

**0**

votes

**0**answers

38 views

### Minimize $td-2\sum_{i=0}^{t-1} w_k(i)$ where $w_k(i)$ is the sum of the base-$k$ digits of $i$

Let $K_k^n$ denote the $n$-fold cartesian product of the complete graph on $k$ vertices, and let $[R,T]$ be the edge cut, consisting of the edges between complementary vertex sets $R$ and $T$.
I ...

**3**

votes

**2**answers

394 views

### summation of products of combinatorials

For any natural number $N$ and $0\le n\le N$ define
$$
f(n) = f(n,N) = \frac{1}{(N+1)!} \sum_{\substack{{S\subset \{1,\ldots,N\}} \\ {|S|=n}}} \prod_{s\in S} s.
$$
(The empty product is interpreted ...

**1**

vote

**1**answer

242 views

### Known results on cyclic difference sets

Is there any infinite family of $v$ for which all the $(v,k,\lambda)$-cyclic difference sets with $k-\lambda$ a prime power coprime to $v$ have been determined?
A subset $D=\{a_1,\ldots,a_k\}$ of ...

**3**

votes

**1**answer

514 views

### Estimate the sum $\sum_{k=1}^n \frac{2^k}{k}$ [closed]

I am concerned with the following sum
$$\sum_{k=1}^n \frac{2^k}{k}_{\displaystyle ~.}$$
It seems that sum must be computed by someone, but I do not know.
By the way, I can figure out a power ...

**3**

votes

**0**answers

59 views

### Distribution of the evaluation (at a non-trivial root of -1) of polynomials with small coefficients

When we studied some cryptographic protocol, we came accross the following problem, which seems linked to the uniformity of the residues of small multiplicative subgroups of $\mathbb{F}_q$.
Problem
...

**7**

votes

**2**answers

278 views

### Sets whose elements are mutually “weakly” coprime?

Fix $n$ and $k$. I want a set $S\subseteq\{1,\ldots,n\}$ with the property that for every $x\in S$,
$$\mathrm{gcd}\bigg(x,\prod_{y\in S\setminus\{x\}}y\bigg)<\frac{x}{k}.$$
How small should a ...

**3**

votes

**1**answer

267 views

### A partition problem

This question is motivated by an old math contest problem, and is a generalization of the original problem. I will write out the original problem as motivation.
Let us say that $n$ is $p$-Savage, for ...

**1**

vote

**0**answers

73 views

### Quadratic transformation of hypergeometric function 2F1

I want to know whether there is some transformation between
$_2F_1(a,b;c;x)$ and $_2F_1(a',b';c';x(1-x))$.
Here is an example called the Kummer quadratic transformation, which may be known to most of ...

**21**

votes

**3**answers

2k views

### How many different numbers can be obtained as product of first $n$ natural numbers?

Let m and n be natural numbers, and consider the set of all possible products of m (not necessarily distinct) elements from the set $\{1,2,\ldots,n\}$, that is consider the set
$\{1^{a_1} \cdot ...

**4**

votes

**1**answer

427 views

### a conjecture in sum-free sets

Let $ A $ be a set of non-zero integers. Then $A$ contains a sum-free subset $B$ of size $ |B|> \frac{|A|}{3} $ (a result of ErdÅ‘s). It is conjectured that RHS can be improved to $\frac{|A|}{3} ...

**0**

votes

**0**answers

100 views

### Multivariate generating function

I am investigating the perturbation of the Jordan canonical form. In my work I must calculate the number of ways to factor $p^ {n-k} q^k$ where $p$ and $q$ are distinct primes ...

**19**

votes

**1**answer

501 views

### Avoiding multiples of $p$

Let $p$ be a prime number and $P=\{1,2,...,p-1\}$
In how many ways we can sum all the elements of $P$ in such a way that we will reach a multiple of $p$
only when we sum the last summand?
For ...

**8**

votes

**3**answers

409 views

### On the vanishing of the generalized von Mangoldt function $\Lambda_k(n)$ when $n$ has more than $k$ prime factors

It is a well-known fact that the generalized von Mangoldt function, defined by
$$\displaystyle \Lambda_k(n) = \sum_{d | n} \mu(d) \left(\log \frac{n}{d}\right)^k$$
vanishes whenever $n$ has more ...

**16**

votes

**1**answer

619 views

### A Question on 1, 2 ,3 Conjecture

1, 2, 3 conjecture is well-known:
If $G$ is a simple graph which is not $K_2$ then one can assign a number among $1, 2, 3$ to every edge such that if we label each vertex with the sum of the numbers ...

**3**

votes

**1**answer

278 views

### Combinatorial Technique Needed

The following problem is likely too special for MO.
However I have no clue how to deal with it, so I'll just try. Nevertheless
it is a combinatorial problem and a discussion about general methods
in ...

**0**

votes

**0**answers

141 views

### special values of L-functions cohen-lenstra heuristic

I found some lecture notes on links between number theory and random permutations. It was difficult to follow:
The notes start with an interesting fact, whose proof I've asked on Math.StackExchange:
...

**16**

votes

**2**answers

932 views

### Infinitely many $N$ such that $\langle p\rangle=\langle q\rangle$ mod $N$

Suppose that $p,q>1$ are two relatively prime integers. Are there infinitely many positive integers $N$ such that
$N$ is relatively prime to $p$ and $q$;
there exists positive integers $k,l$ such ...

**11**

votes

**1**answer

239 views

### A Product Related to Unrestricted Partitions

Start with the product for unrestricted partitions:
(1+x+x$^2$+...)(1+x$^2$+x$^4$+...)(1+x$^3$+x$^6$+...)...
Now replace some of the plus signs with minus signs and expand the product into a ...

**5**

votes

**0**answers

199 views

### 2-adic Logarithm and Resistance of n-dimensional Cube

Resistance across opposite vertices of n-dimensional cube with each edge at one ohm resistance is
$$R_n=\sum_{k=0}^{n-1}\frac1{(n-k){n\choose k}}=\frac1n\sum_{k=1}^{n}\frac1{{n-1\choose k}}.$$
The ...

**2**

votes

**2**answers

218 views

### Irreducible Polynomials from a Reccurence

This question is inspired by a recent one : Let $c$ be a variable and define a sequence by $a_0=0$ $a_1=1$ and $a_{n+1}=a_{n}c-a_{n-1}$ . So
$$\begin{align*}
a_2 &= c
\\ a_3 &={c}^{2}-1= ...

**1**

vote

**1**answer

538 views

### The smallest altitude amongst the triangles formed by points in the unit circle

Let $S$ be a finite set of points inside the unit circle. Consider all possible triangles formed by three distinct points in $S$, and among all such triangles find the smallest altitude. Denote this ...