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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.
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 …
- 156k
6
votes
If two functions are equal to their Newton series, is their composition also equal to its Ne...
Another counter-example is extractable from Gerald Edgar's answer to this question, where he shows that $\sin (ax)$ is discrete analytic for $a \in (-\pi/3, \pi/3)$. So take $
f(x) = \sin ((\pi/4) x)$ …
- 156k
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
8
votes
Accepted
Defining $\{a_i\}$ as $(1+x+⋯+x^k)^n =\sum_{i=0}^{kn}a_ix^i$, then is the 'special' differen...
UPDATE I now have a complete proof. I'll start with the Pascal's triangle case.
For any sequence of real numbers $r=(r_1, r_2, \ldots, r_k)$, define the number of sign changes of $r$ as follows: Dele …
- 156k
4
votes
Can an infinite sequence of integers generate integer-area triangles?
I'll throw out a dumb idea: can anyone find a rational point on
$$y^2 = - (x^2-x+1)(x^2+x+1)(x^2-x-1)(x^2+x-1)?$$
UPDATE: The above formula used to have a sign error, which I have just fixed, and Bj …
- 156k
1
vote
Seeking for a formula or an expression to generate non-repeatative random number ..
I haven't used EXCEL in forever, but here is how I would implement the Knuth shuffle in what I would think of as a generic spreadsheet. I want two 1-dimensional arrays, which I will call $x[i]$ and $y …
- 156k
6
votes
Is $ \sum\limits_{n=0}^\infty x^n / \sqrt{n!} $ positive?
This comment serves to record a partial attempt, which didn't get very far but might be useful to others. Following a suggestion of Mark Wildon and Arthur B, define
$$f_n(\alpha) := \sum (-1)^r \binom …
10
votes
A possible surprise involving Euler's constant $e$
Sure.
$$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}.$$
That prov …
- 156k
26
votes
Accepted
"Are you taller than the average of those who are taller than the average?"
As in Nate's answer, we are interested in iterating the function
$$G(y) := \frac{ \int_{y}^{\infty} x e^{- x^2} dx}{\int_{y}^{\infty} e^{- x^2} }.$$
The numerator is $e^{-y^2}/2$ (elementary). The de …
- 156k
20
votes
Does this sequence always give an integer?
This is the special case $(p,q,r)=(1,2,3)$ of the $3$-term Gale-Robinson recurrence:
$$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}$$
Fomin and Zelevinsky proved that, …
- 156k
8
votes
When is there a closed form for $\sum_{n=1}^{\infty} \frac{P(n)}{Q(n)}$?
As GH from MO says, it depends what you think is a closed form.
Any function of the form $P(n)/Q(n)$ with $\deg P \leq \deg Q -2$ can be written as a linear combination of $1/(n+1) - 1/(n+\alpha)$ an …
- 156k
4
votes
Extremal problem for sequences
The ratio $\sqrt{2}/2$ is optimal. Set
$$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
1
vote
Accepted
A complex sequence with positive values
No.
Consider the matrix
$$\begin{pmatrix}
7 & 8 & 4 & 4 & 0 \\
0 & 7 & 4 & 4 & 0 \\
4 & 4 & 7 & 8 & 0 \\
4 & 4 & 0 & 7 & 0 \\
0 & 0 & 0 & 0 & 5 \\
\end{pmatrix}$$
The eigenvalues are $3 \pm 4i$, $11 …
- 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
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