Coefficients involved in the Binomial Theorem. $\binom{n}{k}$ counts the subsets of size $k$ of a set of size $n$.

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Show that the binomial series satisfies: $(1+x)f'(x)=\alpha f(x)$

If $f(x) = \sum_{n=0}^{\infty}{\alpha\choose n}x^n$, show (without assuming the results of the binomial theorem) that $$(1+x)f'(x)=\alpha f(x)$$ for $|x|<1$ I've already shown that the sum ...
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4answers
59 views

Simplifying a fraction with binomial coefficients [closed]

I'm trying to do a simple combination but seem to forget the shortcut. It is $${\binom{6}{2}+\binom{4}{2} \over \binom{10}{2}}$$ Now finding the answer on my calculator is easy, the problem is that I ...
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1answer
28 views

q-binomial Identity

Unfortunately I am not able to solve the following problem: I tried finding a bijection similar to the prove of this binomial identity: $$\binom{n}{m}\binom{m}{k} = \binom{n}{k}\binom{n-k}{m-k}$$ ...
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7answers
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How to show $\sum_{i=1}^{n} \binom{i}{2}=\binom{n+1}{3}$?

Show that $\,\displaystyle\sum_{i=1}^{n} \binom{i}{2}=\binom{n+1}{3}$. I'm thinking right now (though not getting anywhere with it) that I want to expand out the summation portion to ...
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0answers
13 views

Finding the Value of Ties

THE CHALLENGE If "Winning all the time = 1" "Losing all the time = -1" "Tie all the time = 0" What value would "Winning HALF the time" be? Is there an error in my original variables that would help ...
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5answers
615 views

Given $n \in \mathbb N$, prove $\sum^n_{k=0}(-1)^k {n \choose k} = 0$

I tried to solve it using induction, but that got me no were, in the middle of the equation stat appearing ks that I don't see how to get out of the equation. I think the easiest way to prove it, it's ...
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5answers
95 views

Evaluate $ \lim_{n\to\infty} \frac{(n!)^2}{(2n)!} $ [duplicate]

I'm completely stuck evaluating $ \lim_{n\to\infty} \frac{(n!)^2}{(2n)!} $ how would I go about solving this?
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1answer
267 views

a combinatorial exercise

The problem asks us to calculate: $$ \sum_{i = 0}^{n}(-1)^i \binom{n}{i} \binom{n}{n-i}$$ The way I tried solving is: The given sum is the coefficient of $x^n$ in $ (1+x)^n(1-x)^n $, which is $ (1 ...
1
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1answer
39 views

Generating the Nth combination of a binomial coefficient

I'm designing a protocol, and need a bit of help. I am able to neatly condense the problem I am having into a allegory, I hope it doesn't sound too contrived. Alice has flipped a coin ...
0
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1answer
25 views

Probability and Statictic / Binomial

The cost of a trial conducted in the research and development center of an industrial establishment is known to be 1 million dolars. If the test is negative, in addition to this a new trial is ...
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2answers
74 views

Roots of a polynomial whose coefficients are ratios of binomial coefficients

Prove that $\left\{\cot^2\left(\dfrac{k\pi}{2n+1}\right)\right\}_{k=1}^{n}$ are the roots of the equation $$x^n-\dfrac{\dbinom{2n+1}{3}}{\dbinom{2n+1}{1}}x^{n-1} + ...
4
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1answer
97 views

A convolution involving binomials

Given $$f(i)\gt0,\:g(i)>0,\:i =0,1,2,3,...\:$$and$$\sum_{i=0}^{\infty}f(i) = 1,\sum_{i=0}^{\infty}g(i) = 1$$Prove that, if$$\frac{g(l-k)f(k)}{\sum_{i=0}^{l}f(i)g(l-i)}=\binom{l}{k}p^k(1-p)^{l-k}\: ...
2
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1answer
31 views

Questions on integer-valued polynomials

An integer-valued polynomial or numerical polynomial is a polynomial $f \in \mathbb Q[x]$ with the property that $f(\mathbb Z)\subseteq \mathbb Z$. The set of numerical polynomials forms a subring ...
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2answers
30 views

Proving a sequence of numbers in binomial

Consider the set $P_r={n\choose r}p^r(1-p)^{n-r}$ Prove that: $$\sum_{r=1}^nrP_r=np$$ By far I attempted: $$\sum_{r=1}^nr{n\choose r}p^r(1-p)^{n-r}=\sum_{r=1}^nn{n-1\choose r-1}p^r(1-p)^{n-r}$$ ...
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3answers
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Proving that $r{n \choose r}=n{n-1\choose r-1}$

For proving that: $r{n \choose r}=n{n-1\choose r-1}$ I attempted it with: $r{n\choose r}=\frac{rn!}{r!(n-r)!}=\frac{n!}{(r-1)!(n-r)!}$ $n{n-1\choose ...
3
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2answers
77 views

Formal power series coefficient problem

Find the coefficient of: $[x^{33}](x+x^3)(1+5x^6)^{-13}(1-8x^9)^{-37}$ I have figured out that I need to use this identity: $(1-x)^{-k} = \sum\limits_{i>=0} \binom {n+k-1} {k-1} x^n $ But I ...
6
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2answers
220 views

What is $\lim_{n\to\infty} \sum_{k=0}^{\lfloor n/2 \rfloor} \binom{n}{2k}\left(4^{-k}\binom{2k}{k}\right)^{\frac{2n}{\log_2{n}}}\,?$

What is $$\lim_{n\to\infty} \displaystyle \sum_{k=0}^{\lfloor n/2 \rfloor} \binom{n}{2k}\left(4^{-k}\binom{2k}{k}\right)^{\frac{2n}{\log_2{n}}}\,?$$
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5answers
95 views

Calculate $\lim_{n\to\infty}\binom{2n}{n}2^{-n}$

I would like to show that: $$\lim_{n\to\infty}\binom{2n}{n}2^{-n} = \infty$$ I have gotten as far as: $$ \binom{2n}{n}={(2n)!\over ...
6
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3answers
241 views

A question about limit $\lim_{n\to\infty} {n \choose n/2}/2^n$

My question is: What is the result of this limit: $\displaystyle \lim_{n \to +\infty} \frac{{n \choose n/2}}{2^n}=$ ?
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2answers
59 views

In how many ways can you split six persons in two groups?

In how many ways can you split six persons in two groups? I think that I should use the binomial coefficient to calculate this but I dont know how. If the two groups has to have equal size, then ...
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0answers
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Closed form for $\sum _{k=r}^s \binom{n}{k}$

The cardinality of the Powerset is $2^n$. Looking for $\sum _{k=r}^s \binom{n}{k}$, Mathematica gives $$\sum _{k=r}^s \binom{n}{k}=\binom{n}{r} \, _2F_1(1,r-n;r+1;-1)-\binom{n}{s+1} \, ...
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3answers
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If $p$ is a prime number greater than $2$ and $k$ is a natural number so that $k<p$, how can I prove that?

If $p$ is a prime number greater than 2 and $k\in \mathbb{N}$ so that $k < p$, how can I prove that $p\choose k$ is congruent to $0 \bmod p$?
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1answer
25 views

Show that $(m-1)^{n-1}*n^{m-2}=\binom{n+m-1}{n}$ [closed]

I need help proving that $(m-1)^{n-1}*n^{m-2}=\binom{n+m-1}{n}$.
0
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1answer
43 views

Does this summation (involving binomial) have a closed form? If so, what is it?

The following sums are the ones I'm interested in: $\sum_{i=m}^{\Omega}{\binom{i}{m}i^{-k}}$ $\lim_{\Omega\rightarrow\infty}{\sum_{i=m}^{\Omega}{\binom{i}{m}i^{-k}}}$ I already know that ...
6
votes
4answers
91 views

$\binom{n}{r}$ versus $^n\mathrm{C}_r$ : which notation is more used?

I know that the notation $\binom{n}{r}$ is more standard to use since we have a $\LaTeX$ command for it while there is no such thing for $^n\mathrm{C}_r$. Now, I'm wondering which notation do people ...
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1answer
43 views

binomial distributions and their transforming (6.37-6.39)

I'm lost and frustrated. I don't know how the author (Karl Sigmund; The Calculus of Selfishness) transforms 6.37 in the book pages imaged below: $$ P_y = \sigma w^{N-1} + ...
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1answer
17 views

Approximation Error of Stirlings Formula

Stirlings Approximation : $n! \approx \sqrt{2\pi}n^{n+\frac{1}{2}}e^{-n}$. So $100!$ has an approximate percentage error of about $\frac{100}{12n} = \frac{1}{12}$. Using this information, how does ...
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1answer
34 views

prove an identity involving beta function and gamma function

We know that $B(p,q)=\Gamma(p)\Gamma(q)/\Gamma(p+q)$ where $p, q>0$, and $B(p,q)$ is related to binomial coefficients if one of $p,q$ is an integer. I want to prove the following identity. ...
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2answers
81 views

The value of ${\sum_{k=0}^{20}}(-1)^k\binom{30}{k}\binom{30}{k+10}$

$\newcommand{\b}[1]{\left(#1\right)} \newcommand{\c}[1]{{}^{30}{\mathbb C}_{#1}} \newcommand{\r}[1]{\frac1{x^{#1}}}$ The value of $$\sum_{k=0}^{20}(-1)^k\binom{30}{k}\binom{30}{k+10}$$ It is also the ...
2
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2answers
59 views

Trinomial Theorem for negative exponents

I just learned of binomial theorem for negative integers (or in that case any real $n$). Does such a theorem exist for the trinomial theorem $$(a+b+c)^n$$ and has there been work done? I would think ...
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1answer
787 views

binomial calculation method

I want solve this probability: For $p= 0.4$ $q=0.8$ $n= 20$ $1-P(5<x<11)$ = $1-\sum_{k=6}^{10} \binom{20}{k}(0.4)^k(0.6)^{20-k}. Wolfram Alpha -> = 0,2531$ Is calculation method ...
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2answers
369 views

This question involves Pascal's Triangle & Binomial Theorem. Full Question

Could anyone please help me on the following problem: Factorize the expression $P(n)=n^x-n$ for $x=2,3,4,5$ Determine if the expression is always divisible by the corresponding $x$. If divisible use ...
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2answers
45 views

Combinatorial Proof of falling factorial and binomial theorem

For $n,m,k \in \mathbb{N}$ is true: $$(n+m)^{\underline{k}}=\sum^{k}_{i=0}\binom ki \cdot n^{\underline{k-i}} \cdot m^{\underline{i}}$$ I can prove the binomial theorem for itself combinatorically ...
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2answers
53 views

Proving a rule of combination [duplicate]

I have this question: Considering $$(1+x)^n(1+x)^n=(1+x)^{2n}$$ and show that: $${n \choose 0}^2+{n \choose 1}^2+{n\choose 2}^2+....+{n\choose n}^2={2n\choose n}$$ I have attempted so far: ...
3
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2answers
37 views

show $\sum_{k=0}^n {k \choose i} = {n+1 \choose i+1}$

show for n $\geq i \geq 1 : \sum_{k=0}^n {k \choose i} = $ ${n+1} \choose {i+1}$ i show this with induction: for n=i=1: ${1+1} \choose {1+1}$ = $2 \choose 2$ = 1 = $0 \choose 1$ + $1 \choose 1$ = ...
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votes
2answers
64 views

Find the coefficient of $x^6$ [closed]

I am practicing the Binomial theorem but now I am stuck over this question. Find the coefficient of $x^6$ in $(1 + x^2 + x^4 + x^6)^{20}$ Please help. Thank you
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4answers
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Why the $GCD$ of any two consecutive fibonnaci numbers is $1$?

Note: I've noticed that this answer was given in another question, but I merely want to know if the way I'm using could also give me a proof. I did the following: $$F_n=F_{n-1}+F_{n-2} \\ ...
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3answers
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Probability of 5 cards drawn from shuffled deck

Five cards are drawn from a shuffled deck with $52$ cards. Find the probability that a) four cards are aces b) four cards are aces and the other is a king c) three cards are tens and ...
3
votes
3answers
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Permutations containing a given subsequence

Let $f(n)$ denote the number of $4n$-long strings formed from $2n$ a's and $2n$ b's, such that the string contains, as a (possibly non-consecutive) subsequence, a pattern containing $n$ a's and $n$ ...
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0answers
23 views

how to estimate $\sum (n,i)$?

How to estimate $\sum\limits_{i=-A}^{A} \binom{n}{i}$ ? This probably relates to the central limit theorem and the proof of it. But I want good estimates, also for small values (not just limits to ...
2
votes
0answers
22 views

Equation with $q$-binomial coefficients [migrated]

Let $d\ge2$, and let $q$ be a power of a prime. As usual, define $N(d,q)=\sum_{k=0}^d{d\choose k}_q$. I wonder if there are $d$ and $q$ as above such that $1+N(d,q)=q^{d+1}$. (If the answer is ...
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1answer
33 views

Finding the value of an expression involving co-efficients in binomial expansion

Let $(1+x)^n = C_0 +C_1.x+C_2.x^2 +\ldots+C_n.x^n,$ $n$ being a positive integer. Then find the value of the following expression: ...
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1answer
44 views

Generalisation of Binomial Coefficient (Combinatorics on words)

So, when trying to find subwords from a bigger word: $\binom{abracadabra}{ab} = 5$ with $ABracadabra$, $AbracadaBra$, $abrAcadaBra$, $abracAdaBra$, $abracadABra$. I have noticed that it doesn't go ...
4
votes
2answers
483 views

No closed form for the partial sum of ${n\choose k}$ for $k \le K$?

In Concrete Mathematics, the authors state that there is no closed form for $$\sum_{k\le K}{n\choose k}.$$ This is stated shortly after the statement of (5.17) in section 5.1 (2nd edition of the ...
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1answer
38 views

Show by committee selection argument

First post in Stack Exchange and feel bad to be in need of help. But, I'm having a hard time understanding this one or rather showing the argument. $\binom{n}{k} = \binom{n-2}{k-2} + ...
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2answers
92 views

Sum of series with binomial

How to calculate $$\sum_{n=0}^{\infty}\binom{2n}{n}\frac{2n}{2^{2n}(2n-1)}$$ ? I tried to use residues, generating function, combinatorics formulas, but unsuccessfully.
5
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3answers
169 views

Series of inverses of binomial coefficients

Can you think of a simple way of proving that $$ \sum_{n=k+1}^\infty \frac{1}{n \choose k} $$ is rational for any $k \geq 2$? Here's the background. Consider a series: $$ \sum_{n=1}^\infty ...
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4answers
124 views

Closed Form Expression of sum with binomial coefficient

I have the following equation which is making me problems. $$A_{n} = \sum_{k=0}^{n} \binom{n-k}{k}(-1)^{k}$$ where $n\in\mathbb{N}$. The task is to find a closed form expression for $A_{n}$. I have ...
2
votes
2answers
349 views

Looking for combinatorial identity: $\sum\limits_{j=0}^k{n \choose k-j}{m \choose j}$ [duplicate]

Is there a nicer closed form expression for the following expression? $$\sum_{j=0}^k{n \choose k-j}{m \choose j}$$
1
vote
3answers
25 views

Calculating the sum of coefficients for binomial expression

The question is: Calculate the sum of the coefficients of $(a-b)^{250}$. My reasoning was that we can take the example of $(a-b)^2$, which would have the coefficients of $1$, $-2$, and $1$, ...