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Results tagged with sequences-and-series
Search options answers only
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user 297
for questions about sequences and series, e.g. convergence, closed form expressions, etc. Note that there is a different tag for spectral sequences, and also note that MathOverflow is not for homework. Please consider consulting the online encyclopedia for integer sequences, if you are trying to identify a given sequence that you have found in your research.
8
votes
Slick proof of Stirling's Formula?
I've played around with this a bit. I have a slick lower bound, but not a slick upper bound.
We start with the $ \Gamma $-integral:
$$
n! = \int_{x=0}^\infty x^n e^{-x} dx = \int_{y=-n}^\infty (n+y)^n …
6
votes
Accepted
Powers of $2$ up to $2^{m-1}$ from a polynomial of degree $m-1$
This is a fun problem! You start out by describing the conjectured coefficients of $P(n,m)$, but presumably you started out by computing the polynomial which interpolates $2^{n-1}$ and then noticed a …
- 156k
9
votes
Reciprocals of Fibonacci numbers
Just to make the obvious comments:
Let $q = (1 - \sqrt{5})/2$. Then $F_n=((-q^{-1})^{n} - q^n)/\sqrt{5}$. So
$$\sum \frac{1}{F_n} = \sqrt{5} \sum \frac{q^n}{1-(-1)^n q^{2n}}.$$
This looks kind of like …
- 4,713
31
votes
Elementary proof of the equidistribution theorem
There is a really easy proof that just uses the Pigeonhole principle. Let $\alpha$ be irrational.
Lemma: For any $\delta >0$, there is an $n>0$ such that $(n \alpha) \in (- \delta, \delta) \setminus \ …
- 3,274
3
votes
On the Dirichlet series for $1/\zeta(s)$ for real $s$ and the zeros of zeta
Let $s_0>0$. The right statement is that the following are equivalent:
The sum $\sum_{n=1} \tfrac{\mu(n)}{n^s}$ converges for $s>s_0$.
$\zeta(s)$ has no zeroes with real part $>s_0$.
$1/\zeta(s)$ h …
- 156k
10
votes
A possible surprise involving Euler's constant $e$
Sure.
$$c_n = \frac{1}{n+1} + \frac{1}{(n+1)(n+2)}+ \frac{1}{(n+1)(n+2)(n+3)} \cdots$$
so
$$\frac{1}{n+1} < c_n < \frac{1}{n+1}+\frac{1}{(n+1)^2}+\frac{1}{(n+1)^3}+\cdots = \frac{1}{n}.$$
That prov …
- 156k
9
votes
Optimal Talmudic Zigzag
This is a special case of finding the longest path in a directed acyclic graph. Namely, the vertices of our graph are $1$, $2$, ..., $n$, there is an edge $i \to j$ for each $i<j$ and the length of th …
- 156k
24
votes
Rearrangements that never change the value of a sum
For the purpose of recording an answer rather than just a pile of links: Michael Hardy requires that
if either limit exists then so does the other and in that case then they are equal?
Let's cal …
- 4,713
7
votes
Accepted
Angle between two given vector is small. Can we permute coordinates of them such that new ve...
No. Let
$$\vec{x} = \vec{y} = \frac{1}{\sqrt{12+6 \sqrt{3}}} (-2-\sqrt{3},1,1+\sqrt{3}).$$
So $\vec{x} \cdot \vec{y} = 1$, but $x_1 y_2+x_2 y_1 + x_3 y_3=0$.
- 156k
4
votes
Extremal problem for sequences
The ratio $\sqrt{2}/2$ is optimal. Set
$$a_n = \binom{n-7/4}{n-1} r^n \ \mbox{for} \ n \geq 1.$$
Let $Y(r)$ and $X(r)$ be the corresponding sums.
I claim that
$$ \lim_{r \to 1^-} \frac{X(r)}{Y(r)} = …
- 156k
10
votes
The coefficient of a specific monomial of the following polynomial
The coefficient is always zero. I use the superb conventions of Concrete Mathematics: Sums are over all integers unless otherwise indicated, and $\binom{n}{k}$ is $0$ if $k<0$ or $>n \geq 0$.
Expandi …
- 33.8k
8
votes
When is there a closed form for $\sum_{n=1}^{\infty} \frac{P(n)}{Q(n)}$?
As GH from MO says, it depends what you think is a closed form.
Any function of the form $P(n)/Q(n)$ with $\deg P \leq \deg Q -2$ can be written as a linear combination of $1/(n+1) - 1/(n+\alpha)$ an …
- 156k
6
votes
Accepted
What is the rate of convergence?
Putting $x_n = 1+y_n$, we have $y_{n+1} = y_n + \frac{y_n^2}{2}$. There is a standard method for analyzing recursions of the form $y_{n+1} = y_n+O(y_n^2)$. A good introduction is chapter 8.4 in de Bru …
- 14k
4
votes
Accepted
On one class of Somos-like sequences
I believe this is a special case of Case (9) in Theorem 3.9 of Allman, Cuenca and Huang. By the way, this paper was an REU project!
- 156k
20
votes
Does this sequence always give an integer?
This is the special case $(p,q,r)=(1,2,3)$ of the $3$-term Gale-Robinson recurrence:
$$x_{n+p+q+r} x_n = x_{n+p} x_{n+q+r} + x_{n+q} x_{n+p+r} + x_{n+p+q} x_{n+r}$$
Fomin and Zelevinsky proved that, …
- 12.3k