For questions about the evaluation of finite products, or their properties. For infinite ones, use "infinite-products" tag.

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2
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0answers
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What is $\prod _{j=1}^n \left(\sqrt{j}+1\right)$?

By the Fundamental Theorem of Algebra, it is easily seen that for a monic polynomial $p(x) \in \mathbb{C}[x]$, $$\prod _{j=1}^n p(j) = \frac{\prod_{p(r)=0}\Gamma(1+n-r)}{\prod_{p(r)=0}\Gamma(1-r)},$$ ...
1
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0answers
14 views

nth product of sequential matrices

$\forall n \in \mathbb{N}$, let: $$P_n = \left( \begin{matrix} a & 1-a \\ b_n & 1-b_n \end{matrix} \right). $$ Whereby $\{b_n\}_{n \in \mathbb{N}}$ is a monotonically increasing sequence of ...
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1answer
32 views

Value of a product of cosines and the floor of its reciprocal

$$ \mbox{The question states}\quad {a \over b} =\prod_{n = 1 \atop{\vphantom{\LARGE A}n \not= 9}}^{17}\cos\left(n\pi \over 18\right) $$ $$\mbox{And it is also provided that}\quad \left\lfloor{b \over ...
1
vote
1answer
34 views

Box topology is finer than the uniform topology on $\mathbb{R}^\mathbb{N}$

This time, I wish to show that the box topology is finer than the uniform topology on countable Cartesian products on $\mathbb{R}$ denoted $\mathbb{R}^\mathbb{N}$ However, the problem here is that ...
1
vote
1answer
27 views

Uniform topology is finer than the product topology on $\mathbb{R}^\mathbb{N}$

I wish to show that the uniform topology is finer than the product topology on countable Cartesian products on $\mathbb{R}$ denoted $\mathbb{R}^\mathbb{N}$ We know both spaces are metrizable: The ...
-2
votes
2answers
50 views

Why $\prod_{i=1}^{k-1} \left( 1 - \frac{i}{N}\right) = 1 - \frac{ k \choose 2 }{N}$ [closed]

In this biology textbook, they show the following equation: $$\prod_{i=1}^{k-1} \left( 1 - \frac{i}{N} \right) = 1 - \frac{ k \choose 2 }{N}$$ where both $N$ and $k$ are positive integers and $k &...
5
votes
2answers
63 views

About “The product of the six numbers surrounding any interior number in Pascal’s triangle is a perfect square”

The current Futility Closet has this statement: "The product of the six numbers surrounding any interior number in Pascal’s triangle is a perfect square." Here is the link with a nice illustration: ...
1
vote
2answers
42 views

Relation between HCF, LCM and product of multiple numbers [duplicate]

It is well known that for two numbers $a $ and $b$, $$\text {lcm} (a,b)\times \text {hcf} (a,b)=ab$$ Does there exist a similar equality/ inequality between HCF, LCM and product of multiple numbers? (...
2
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0answers
42 views

Multiplication of polynomials of the same degree

Consider polynomials of the form \begin{equation} p(x)=x^{n-2r}\sum_{i=0}^ra_ix^{2i}, \end{equation} where \begin{align} r&=n/2, \quad n \quad \text{even},\\ r&=(n-1)/2, \quad n \quad \text{...
56
votes
3answers
4k views

Does multiplying all a number's roots together give a product of infinity?

This is a recreational mathematics question that I thought up, and I can't see if the answer has been addressed either. Take a positive, real number greater than 1, and multiply all its roots ...
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votes
1answer
52 views

Is this matrix going to be real or complex?

I hope that this is the right forum where to post this question (and not here). I have a Chi-Square Kernel Matrix (using the second version, which is positive-definite) ...
0
votes
1answer
38 views

Is it possible to calculate the Average of Products from the Product of Averages?

I have two sets of data - one $X$ measuring the unavailability at a site, the other $Y$ measuring the number of antennas at each site. I want to calculate the overall average unavailability as the ...
0
votes
1answer
45 views

Finding the product of a prime function…

If we take the primes $p_k < n$, and raise them to the highest power possible such that $(p_k)^{r_k} \le n$, what is the lower bounds on $\prod{ (p_k)^{r_k} }$? In other words, what are the ...
0
votes
1answer
24 views

Composition method and constructing a relation.

Let $R = \{(1, 5), (2, 2), (3, 4), (5, 2)\}$, $S = \{(2, 4), (3, 4), (3, 1), (5, 5)\}$, and $T = \{(1, 4), (3, 5), (4, 1)\}$. Find (1)$\quad R ∘ S$ (2)$\quad T ∘T.$ (3) $\quad T∘S$ ...
1
vote
1answer
28 views

Fibred product of sets $X\times_Z Y=\{(x,y)\in X\times Y: \alpha(x)=\beta(y)\}$ satisfies the universal property.

This is Exercise 1.3.N from Vakil's notes of Algebraic Geometry. The following is the diagram defining the universal property of fibred product: Show that in $\mathit{Sets}$, $$X\times_Z Y=\{(x,...
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votes
10answers
8k views

Is $0! = 1$ because there is only one way to do nothing?

The proof for $0!=1$ was already asked at here. My question, yet, is a bit apart from the original question. I'm asking whether actually $0!=1$ is true because there is only one way to do nothing or ...
3
votes
3answers
69 views

what does $(A\cdot\nabla)B$ mean?

I was studying a physics book and I saw this expression $$(A\cdot\nabla)B$$ where $A$ and $B$ are vectors. What's the definition of this? I've also seen this in some identities
0
votes
0answers
26 views

How to estimate the product of the $k$ largest eigenvalues of a matrix

Now I have a question which let me to prove that the product of the largest $k$ singular values of a real matrix is always larger than the one of $k$ largest eigenvalues. For $k=1$, I use the ...
2
votes
1answer
61 views

Value of finite product based on empty set

How does one evaluate the following product if the set S happens to be empty? \begin{aligned} f(n)= n \prod_{x \in S} \left(1-\frac{1}{x}\right) \end{aligned} Is the value simply n or is it ...
0
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0answers
30 views

Linearise product of two non-negative variables

Is there a trick to linearise the product of two non-negative (decision) variables in linear optimisation? Let $x_1$ and $x_2$ be these variables with $0 \leq x_1 \leq a$, $a \in \mathbb{R}_+$ and $...
0
votes
1answer
27 views

Is there anything known in general about upper and lower bounds for $\prod_{i\leq n\vee p_n>k}(p_i-k)$

I have no specific reason to ask this question other than seeing that it comes up quite often when I'm playing around with prime numbers. Let $$f(n,k)=\prod_{i \leq n\vee p_n>k}(p_i-k)$$ Where $p_i$...
1
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1answer
24 views

Understanding components of a vector

I learned that we can get the component of a vector in any direction using the dot product. The problem I have is the meaning of the term component itself. The component of a vector $\vec A$ in the ...
7
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1answer
672 views

Product of Fibonacci numbers

I'm looking for the asymptotic approximation of the product of the first $n$ Fibonacci numbers. Does there exist a tight approximation for these kind of things?
0
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1answer
53 views

Product as the sum of powers times a symmetric polynomial: What's the name of this property and what is it used for?

I noticed that the product of a group of positive integers $N$ with $n$ elements can be expressed as the sum of powers of the smallest member of $N$ times some (what I later found out be called) ...
0
votes
1answer
48 views

Proving ${\pi\over 2}=2\tan^{-1}\left({1\over A}\right)+\tan^{-1}\left({1\over B}\right)$

Let $A=2^{2^{-x}}$ and $B=2^{2^{-x}+1}(1+2^{2^{-1}})(1+2^{2^{-2}})\cdots(1+2^{2^{-x+1}})$ Showing $x\ge2$ $${\pi\over 2}=2\tan^{-1}\left({1\over A}\right)+\tan^{-1}\left({1\over B}\right)\tag1$$ ...
4
votes
1answer
63 views

The limit of consecutive positive integers which are the product of n primes.

The maximum length of a string of consecutive primes is 2: that is, the primes 2, 3. This is easily proven, as no even number other than 2 is prime. In contrast, consider the set of numbers which ...
0
votes
1answer
24 views

If two infinite sets have the finite closed topology, then their product does not have the finite closed topology.

Let $X_1$ and $X_2$ be infinite sets and $T_1$ and $T_2$ be the finite-closed topology on $X_1$ and $X_2$, respectively. Show that the product topology, $T$, on $X_2 \times X_2$ is not the finite ...
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1answer
34 views

Source of faulty reasoning in expectation of product of random variables?

For iid $\xi_i>0$, with $\mathbb E[\xi_i]=1$, what is $\mathbb E[\prod_i^M\xi_i]$? Approach 1: $\mathbb E[\prod_i^M\xi_i]=\prod_i^M\mathbb E[\xi_i]=1$. There is another approach for $M\gg1$ with ...
1
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1answer
44 views

Proof of $\sin nx=2^{n-1}\prod_{k=0}^{n-1} \sin\left( x + \frac{k\pi}{n} \right)$

I have seen this identity on Wolfram mathworld and in a comment to another similar trigonometric proof: Prove that $\prod_{k=1}^{n-1}\sin\frac{k \pi}{n} = \frac{n}{2^{n-1}}$ I can't seem to find a ...
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1answer
36 views

Modified Sum of Products

A given number k is to be expressed as a sum of products of integers keeping in mind that the integers used in above process do not exceed their cumulative sum as 100. For e.g., k = 19 can be ...
7
votes
1answer
82 views

Summation is to integration, as the large product operator is to…? [duplicate]

The integral is defined many ways but one that I am aware of is the Riemann Integral(?) which looks sorta like $\sum^n_{i=0} f(a +i\frac {b-a}n)*\frac {b-a} n$. An interesting thought is "is there a ...
0
votes
1answer
22 views

Product of all Square Roots, taken only Decimal Digits

How and where could I compute the decimal reminder of a product of square roots times ten: $$Dr\left( \prod_{x=1}^{k}x^\frac{1}{2} \right) \times 10$$ Where $k$ is a power of $10$. I would like to ...
2
votes
1answer
56 views

How to prove that $e^{-\gamma}=\prod_{n=1}^\infty\left(1+\frac1n\right)e^{-1/n}$

Suppose we defined the Gamma function $$\frac1{\Gamma(z)}=ze^{\gamma z}\prod_{n=1}^\infty\left(1+\frac zn\right)e^{-z/n}$$ where $\gamma$ is just a constant. I want to prove that $\Gamma(1)=1$, so I ...
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1answer
42 views

Prove $2^i\prod_{j=1}^{i}\sum_{k=1}^{2j-1}(-1)^{k-1}k^2=(2i)!$

Prove identities (1) $$2^i\prod_{j=1}^{i}\sum_{k=1}^{2j-1}(-1)^{k-1}k^2=(2i)!$$ (2) $$2^i\prod_{j=1}^{i}\sum_{k=1}^{2j}(-1)^{k-1}k^2=(2i+1)!$$ Is there another approach to prove (1)? $$2^i\...
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1answer
37 views

Why is $\prod_{i=1}^{k-1}\left(1-\frac{i}{2N}\right) \approx 1 - \frac{{k \choose 2}}{2N}$?

I came across this approximation in the book Principles of Population Genetics by Hartl and Clark (page 130). $$\prod_{i=1}^{k-1}\left(1-\frac{i}{2N}\right) \approx 1 - \frac{{k \choose 2}}{2N}$$ ...
1
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0answers
19 views

In which cases are the main diagonal elements of a product of positive definite matrices positive?

Let $A$ and $B$ be symmetric positive definite (pd) $n \times n$ matrices and $C = A \cdot B$. In which cases is then every $c_{ii}$, the $i$-th main diagonal elements of $C$, positive? When $A$ ...
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0answers
16 views

Doing a project for a CALC II class, and need help determining if we have sufficiently solved a product problem.

We finished our final in Calc II, and now we are doing math projects to pass time until the semester ends. https://docs.google.com/presentation/d/1bYMKfCqcc9zG32Zs_Wti2Fp9rXV7Nbt4QxR3Vnp33Dc As you ...
0
votes
1answer
29 views

Spectra of matrices with null product

If I consider two square matrices $A$ and $B$ such that $A B = B A =0$ and I know eigenvalues and eigenvectors of $A$, is it possible to get informations about the spectrum of $B$? In particular, I ...
2
votes
1answer
30 views

Weierstrass Approximation Theorem for a Product Space?

I am faced with the following problem: Let $X$ and $Y$ be compact Hausdorff spaces and $f$ belong to $C(X \times Y)$. Show that for each $\epsilon > 0$, there are functions $f_{1}, f_{2}, \cdots , ...
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0answers
27 views

which values of k satisfies special property to formulate L function

Consider $x*\prod_{a=1}^{n}(1-x^a)^k$ Famously for k=24 this product satisfies the condition to be an L-Function. More information can be found here My question is for what other values of k, such ...
1
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1answer
16 views

Euclidean algorithm for dividing two products.

Say I have numbers, $a$ and $b$ represented as two products $$a = \prod_{i=0}^{N_a} a_i \hspace{1cm} b = \prod_{i=0}^{N_b}b_i$$ I do know $\{a_k\}$ and $\{b_k\}$ but can not store $a$ or $b$ in a ...
0
votes
0answers
24 views

Minimize and maximize the sum of dot products at the same time

this is the problem. I have a set of numerical positive vectors of equal length. For each pair of vectors $(\mathrm{i}, \mathrm{j})$ I define the vector $\mathrm{ij}=\mathrm{i} - \mathrm{j}$. I also ...
0
votes
2answers
37 views

Why is my answer incorrect for this differentiation question?

$$y = x* ((x^2+1)^{1/2})$$ I must find $$dy/dx$$ $$u = x, v = (x^2+1)^{1/2}$$ To do this I must use the product rule and the chain rule. To get dv/dx, $$(dv/dx) = (1/2)*(b)^{-1/2}*2x $$ $$(dv/dx) ...
2
votes
0answers
123 views

What notation would I use to differentiate between a cartesian product and a cotangent bundle of surfaces?

If the $S^1$ is defined by $x^2 + y^2 = r^2$ , $T^2 = S^1 \times S^1$ is defined by $\left(\sqrt{x^2 + y^2} -R\right)^2 + z^2 = r^2$ , $T^3=S^1\times S^1\times S^1$ is defined by $\left(\sqrt{\left(...
4
votes
2answers
83 views

Prove $\prod_{k=1}^n(1+a_k)\leq1+2\sum_{k=1}^n a_k$

I want to prove $$\prod_{k=1}^n(1+a_k)\leq1+2\sum_{k=1}^n a_k$$ if $\sum_{k=1}^n a_k\leq1$ and $a_k\in[0,+\infty)$ I have no idea where to start, any advice would be greatly appreciated!
1
vote
1answer
24 views

Find when the product would be an integer

The problem: The sequence $\{a_n\}$ is defined recursively by $a_0=1,a_1=\sqrt[19]{2}$ and $a_n=a_{n-1}a_{n-2}^2$ for $n \geq 2$. What is the smallest positive integer $k$ such that the product $a_1 ...
1
vote
1answer
45 views

Equivalence of definition of product in a category

I was reading Mitchel book on categories and the following observation without proof is given: A family of morphisms given by $\lbrace p_{i}:A \to A_{i} \rbrace$ is the product of $A_{i}$ if and only ...
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votes
2answers
32 views

By induction prove $\prod _{i=1}^{n} \frac{n+i}{2i-3} = 2^n (1 - 2n)$

I need to prove the following by induction. $\forall n \in \Bbb N$ $\prod _{i=1}^{n} \frac{n+i}{2i-3} = 2^n (1 - 2n)$ I know the steps to take but I'm failing to come to the right side of the ...
0
votes
0answers
13 views

Parentheses and Comma Notation

I came across the following formula for normalizing Smith-Waterman scores, and I do not understand what the SW(p1, p2) part is trying to notate. Does it perhaps refer to a product? Click here to see ...
2
votes
1answer
95 views

Prove that $\prod\limits_{2 < p \leq y}\left(1-\frac{2}{p}\right)\sim\frac{D}{\log ^2 y}$ [duplicate]

I'm writing my bachelor thesis about Brun's sieve method and his theorem. In one proof I found this statement without further explanation. It is important to show that the product doesn't converge "...