Questions on the Fibonacci numbers, a special sequence of integers that satisfy the recurrence $F_n=F_{n-1}+F_{n-2}$ with the initial conditions $F_0=0$ and $F_1=1$.

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10
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4answers
302 views

Linear Combinations of Fibonacci Numbers (integer coefficients)

While working on problem #2 on Project Euler, I came across the need to express $F_n$ as a linear combination of $F_{n-3}$ and $F_{n-6}$. This is relatively simple to do: $$\begin{align} F_n &= ...
32
votes
6answers
5k views

Proof the Fibonacci numbers are not a polynomial.

I was asked a while ago to prove there is no polynomial $P$ in $\mathbb R$ such that $P(i)=f_i$ for all $i\geq0$. I tried to get a proof as slick as possible and here's what I got. Let ...
1
vote
2answers
123 views

Modulus of sum of sequence of Fibonacci numbers

What is the most efficient way to find the modulus of sum of sequence of fibinacci numbers. For example (F(N) + F(N + 1) + ... + F(M)) mod 1000000007.
15
votes
5answers
3k views

Is it true that the Fibonacci sequence has the remainders when divided by 3 repeating?

About this Fibonacci sequence, is it true that the remainders when divided by three repeat along with the sequence like this: Fibonacci sequence: $1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, ...
36
votes
1answer
1k views

Why does this test for Fibonacci work?

In order to test if a number $A$ is Fibonacci, all we need to do is compute $5A^2 + 4$ and $5A^2 -4$. If either of them is a perfect square, the number is Fibonacci, otherwise not. Why does this test ...
53
votes
3answers
606 views

Closed form solution for $\sum_{n=1}^\infty\frac{1}{1+\frac{n^2}{1+\frac{1}{\stackrel{\ddots}{1+\frac{1}{1+n^2}}}}}$.

Let $$ \text{S}_k = \sum_{n=1}^\infty\cfrac{1}{1+\cfrac{n^2}{1+\cfrac{1}{\ddots1+\cfrac{1}{1+n^2}}}},\quad\text{$k$ rows in the continued fraction} $$ So for example, the terms of the sum $\text{S}_6$ ...
0
votes
1answer
98 views

Hyper sum of Fibonacci numbers

Let $F(n)$ be the $n$-th Fibonacci number. That is, $F(n)$ satisfies $F(0)=0,F(1)=1,F(n)=F(n−1)+F(n−2) (if n≥2).$ Let $f_k(n)$ be the function such that $f_0(n) = F(n)$, $f_k(n) = ...
1
vote
2answers
89 views

Divisibility of $987x^n − F_nx^{16} + F_{n−16}$

If $F_n$ is $n^{th}$ Fibonacci number, and polynomials $P_n(x)$ are defined as $987x^n − F_nx^{16} + F_{n−16}$, prove that for all $n ≥ 1$, $P_n(x)$ is divisible by $x^2−x−1$. This is from a ...
20
votes
6answers
531 views

Why does every “fibonacci like” series converge to $\phi$?

It's is well known that the ratio of side-by-side fibonacci numbers converge to $\phi$. But it seems by my calculations, that if one starts with any pair of numbers one will also get a ratio that ...
3
votes
0answers
69 views

Proving generalized Cassini's identity using determinant?

Motivation It is not hard to show, by using the general solution, that Proposition. If $(a_{n})_{n\in\Bbb{Z}}$ satisfies the recursive formula $ a_{n+2} = pa_{n+1} + qa_{n}$, then for any $n, i, ...
1
vote
0answers
23 views

Fibonacci, prove that $F_{n}\cdot F_{n+2}-({F_{n+1}})^2=(-1)^n$ with induction [duplicate]

I need to prove by induction that: $$F_{n}\cdot F_{n+2}-({F_{n+1}})^2=(-1)^n$$ I did the following: Check if the statement holds for $n=1$: $$1\cdot 3-(2)^2=(-1)^1$$ Check if the statement ...
21
votes
1answer
429 views

Fibonacci-related sum

Related to this question Find a solution for f(1/x)+f(1+x)=x, what is this sum: $$\sum_{n=1}^{\infty}(-1)^n\left(\frac{F_n}{F_{n+1}}-\frac1{\phi}\right)$$ where $F_n$ is the $n$th Fibonacci number and ...
1
vote
1answer
54 views

Confusion on unberstanding the proof of induction regarding Fibonacci numbers

I am trying to understand the proof that "For all $n\geq 2, F_n^2-F_{n+1}F_{n-1}=(-1)^{n-1}$.Where $F_n$ stands for the Fibonacci number at $n$. I got this proof from a book and here is the proof. ...
0
votes
3answers
99 views

Matrices, determinants, and applications to identities involving Fibonacci numbers

Preamble It is well known that since: $$ \begin{pmatrix} F_{n+1} \\ F_n \\ \end{pmatrix} = \begin{pmatrix} 1 & 1 \\ 1 & 0 \\ \end{pmatrix} \begin{pmatrix} F_n & F_{n-1} ...
2
votes
2answers
70 views

Seven expressions involving $F_n$ an $L_n$ that are always composite

Prove that if $F_n$ an $L_n$ are Fibonacci and Lucas numbers respectively, and $n>2$, then $$F_{n-2}\times F_{n-1}\times F_{n+1}\times F_{n+2}-15$$ $$F_{n-2}\times F_{n-1}\times ...
6
votes
1answer
129 views

Prove or disprove that ${F_{n}}^2 + 41$ is always a composite

The problem: Prove or disprove: If $F_{n}$ is the $n^{th}$ Fibonacci number then $${F_{n}}^2 + 41$$ is always a composite number. It looks that if $n$ is not multiple of 12, ${F_{n}}^2 + 41$ ...
6
votes
1answer
112 views

Prove or disprove that ${F_{n}^2} + 43$ is always a composite

This is a kind of follow-up to another question, but in order not to burden that question and its answers with new comments, I decided to create this separate question. Also, it looks this problem is ...
37
votes
5answers
847 views

Is ${F_{n}}^2 - 28$ always a composite number?

The problem is as follows: Prove or disprove that if ${F_{n}}$ is $n$-th Fibonacci number, and $n>5$, than $${F_{n}}^2 - 28$$ cannot be a prime. I came across this problem accidentally ...
2
votes
0answers
91 views

Who First Considered This Generalization of the Fibonacci Numbers?

I am looking for the author who originally researched a generalization of the Fibonacci numbers, which Koshy, in Chapter 7 his book Fibonacci and Lucas Numbers with Application refers to as the ...
5
votes
1answer
194 views

Prove Divisibility In Fibonacci Sequence Over A Prime Number

In The Fibonacci sequence which is defined as: Lets say we have the number $p$ which is an odd prime. Prove that: $F_{p-1} + F_{p+1} -1$ Is divisible by $p$. Prove that for any given $n$ real ...
4
votes
1answer
82 views

Summation of a multiple series involving Fibonacci numbers

Compute the sum $$\sum_{a_{2015} = 0}^{\infty} \sum_{a_{2014} = 0}^{a_{2015}} \sum_{a_{2013} = 0}^{a_{2014}} \cdots \sum_{a_{1} = 0}^{a_2} \sum_{k=0}^{a_1} \frac{F_{k}}{2^{a_{2015}}} $$ where $F_k$ ...
7
votes
3answers
87 views

If $f_{n-1}^2=(f_n/2)^2+h^2$ then $n=6$

How can I prove that if $f_n$ is a term of the Fibonacci sequence divisible by $4$ and if $$f_{n-1}^2=(f_n/2)^2+h^2,$$ $h\in\Bbb Z^+$ then $n=6$? I know that since $\gcd(f_k,f_{k+1})=1$ for every ...
0
votes
0answers
50 views

Induction: Fibonacci / Lucas Numbers [duplicate]

From Andrews' Number Theory, Chapter 1, Section 1, Problem 15: Prove, by induction, that $F_{2n} = F_nL_n$ where $F_n$ denotes the nth Fibonacci number and $L_n$ denotes the nth Lucas ...
1
vote
2answers
133 views

Prove that $F_n={n-1 \choose 0 }+{n-2 \choose 1 }+{n-3 \choose 2 }+\ldots$ where $F(n)$ is the $n$-th fibonacci number [duplicate]

If $F_n$ is the $n$-th fibonacci number, then prove that, $$F_n={n-1 \choose 0 }+{n-2 \choose 1 }+{n-3 \choose 2 }+\ldots$$ I tried the idea of using Pascal's triangle, but it seems to need some ...
1
vote
2answers
77 views

A series for Fibonacci numbers.

How can I prove The Fibonacci sequence is encoded in the number $1/89$ i.e. $( 1/89 = 0.01 + 0.001 + 0.0002 + 0.00003 + 0.000005 + 0.0000008 + 0.00000013 + 0.000000021 + 0.0000000034 \ldots)$
4
votes
3answers
229 views

Inequality of the Fibonacci sequence and the golden ratio

How can I prove that for each $n\in\Bbb Z^+$ $$\frac{f_{2n}}{f_{2n-1}}\leq\frac{1+\sqrt{5}}{2}$$ where each $f_i$ is a term of the Fibonacci sequence. Any help is really appreciated
1
vote
2answers
56 views

Find $x$ as the given $n$th term in the Fibonacci sequence?

With a given $n$ and I am trying to find the value of $x$, as in: $$Fib(x)=n$$ Using the formula for Fibonacci sequence, where $\varphi$ is the Golden Ration ($\approx1.61803399\ldots$) $$Fib(z) = ...
2
votes
3answers
74 views

Writing $1+3x^2+8x^4+21x^6+\cdots$ as a power series representation

How would I write the power series $$1+3x^2+8x^4+21x^6+\cdots$$ as a power series representation (something neat similar to $\frac{1}{1-x}$)? This reminds me of the power series ...
1
vote
4answers
62 views

How to prove this equation by induction?

I am trying to prove this equation by mathematical induction $$f_{n+1}f_{n-1} = f_{n}^{2}+(-1)^n$$ is true where $f_{n} = $ the nth number in the Fibonacci sequence. I don't quite get how to do this ...
0
votes
2answers
40 views

Problem on deriving binet formula

I'm trying to understand binet formula. I got a good explanation here. Please look at the link. Everything just fine but one thing. It said that $A_n = A_{n-1} + A_{n-2}$, which is fibonacci. But why ...
2
votes
3answers
89 views

Showing convergence of recursive sequence $A_{n+1}=\frac 1 {1+A_n}$

Given : $\forall n\in\Bbb N,\quad A_{n+1} = \frac 1 {1+A_n}$ and $A_1 = 0$ Show the sequence converges and find its limit. Briefly what I did was to create two sub-sequences with an index ...
1
vote
1answer
44 views

Prove that the set of solutions to $F_{n+2} = F_{n+1} + F_n$ is of dimension 2

I was playing with the Fibonacci sequence, willing to prove that $$ F(n) = \frac{1}{\sqrt{5}}\left(\left(\frac{1+\sqrt{5}}{2}\right)^n - \left(\frac{1-\sqrt{5}}{2}\right)^n\right) $$ I did the usual, ...
1
vote
1answer
40 views

Discreet Math - Given n>= 5 how many times does fib(4) occur?

I have been trying to solve the below problem (and similar problems) but I have no clue how to tackle it. Can please help me tackle this particular problem, and how to attack similar problems? The ...
5
votes
4answers
667 views

Sum of cubes of first n fibonacci numbers

Let $\{f_k\}$ be the sequence of fibonacci numbers. It is well-known that $\sum_{k=1}^n f_k=f_{n+2}-1$ and $\sum_{k=1}^n f_k^2=f_n f_{n+1}$ . Is there a formula for $\sum_{k=1}^n f_k^3$ ?
6
votes
2answers
212 views

Sequence with denominators of products of consecutive Fibonacci numbers

I'm trying to figure out a way to solve the value of this: $$\frac{1}{1\times 2}-\frac{1}{2\times 3}+\frac{1}{3\times 5}-\frac{1}{5\times 8}+\frac{1}{8\times 13}-\dots$$ The only thing I can come up ...
1
vote
4answers
156 views

Prove that the Fibonacci recursion diverges

I have this sequence with $ n \in \mathbb{N} $ $ f(1) = f(2) = 1 $ and $ f(n) = f(n-1) + f(n-2) $ for $n \ge 3$ I think this sequence is bounded below and unbounded above. So it's clear that this ...
1
vote
1answer
39 views

Limit of Ratio of Two Generalized Fibonacci Sequences

I am hoping someone can help me determine the limit of two unique generalized Fibonacci sequences. Most everyone is familiar with the much talked about $\lim_{x\to \infty}$ ...
0
votes
2answers
35 views

Prove by induction fibonacci variation

Prove by induction: The fibonacci sequence is defined as follows: $f_1 = 1$, $f_2 = 1$ and $f_{n+2} = f_n + f_{n+1}$ for $n \geq 1$ Prove by induction that $f_1^2 + f_2^2 + \dotsb + f_n^2 = f_n ...
2
votes
2answers
69 views

Fibonnaci and Lucas series technique

Well I have the following two problems involving fibonnaci sequences and lucas numbers, I know that they share the same technique, but I don't have clear the procedure: $$f_n = f_{n-1} + f_{n-2}: f_0 ...
4
votes
5answers
100 views

Prove that $F(1) + F(3) + F(5) + … + F(2n-1) = F(2n)$

(These are Fibonacci numbers; $f(1) = 0$, $f(3) = 1$, $f(5) = 5$, etc.) I'm having trouble proving this with induction, I know how to prove the base case and present the induction hypothesis but I'm ...
3
votes
1answer
84 views

New Identities for Generalized Fibonacci Numbers?

Over the past few months I have been investigating one the generalizations of the Fibonacci numbers, called the Generalized Fibonacci Numbers (GFNs). The GFNs are just like the regular Fibonacci ...
1
vote
1answer
73 views

Induction proof for Fibonacci numbers

I am trying to get a hang on the induction method for proof, but I'm still dubious of many aspect of this proof regarding its application to sequences of integers, such as the Fibonacci sequence. ...
2
votes
4answers
75 views

Proving this $F_{n+1} \cdot F_{n-1} - F^2_n = (-1)^n$ by induction

Where $n \in \mathbb{N}$ and $$ F_n = \begin{cases} 0 & \text{ if } n = 0 \\ 1 & \text{ if } n = 1 \\ F_{n-1} + F_{n-2} & \text{ if } n > 1 ...
2
votes
1answer
46 views

What does “The closure of the shift-orbit of the Fibonacci word” mean?

Im trying to translate an article about rauzy fractal. But since my English is not good enough I cant understand this paragraph: ...
2
votes
1answer
33 views

How can we be sure of periodicity by testing some terms?

A mod $n$ Fibonacci sequence is simply defined as the Fibonacci sequence, except all terms are in mod $n$. Now to determine periodicity, the worked solutions computed the first 20 or so terms and ...
3
votes
2answers
109 views

How is the Binet's formula for Fibonacci reversed in order to find the index for a given Fibonacci number?

a question about the Fibonacci sequence: $$F_n =\frac{\phi^n-(-\frac{1}{\phi})^n}{\sqrt{5}}$$ This is the Binet's formula for the nth Fibonacci number. if I reverse it I can get: ...
1
vote
2answers
51 views

Proof By Induction Fibonacci Numbers

How do I prove that $$ f_{ 2n+1 } = 3f_{ 2n } + 1 - f_{ 2n-3 } $$ I'm not sure how to prove it using the defining recurrence of Fibonacci numbers.
12
votes
2answers
134 views

Remainders of Fibonacci numbers

Let $a>b$ be positive integers. Is there a Fibonacci number that is $b$ modulo $a$? We know that the Fibonacci numbers are periodic modulo $a$. Indeed, consider pairs $(F_i,F_{i+1})$ modulo $a$. ...
0
votes
3answers
74 views

How to prove the convergence of $\lambda_n = \frac{f_{n+1}}{f_n}$?

A question about the fibonacci sequence. I have a sequence: $$\lambda_n = \frac{f_{n+1}}{f_n}$$ While $f_n$ is the fibonacci sequence. I also have the equation: $$ 0 = x^2 - x -1$$ And i know ...
0
votes
1answer
25 views

Why the number of subsets S ⊂ {1,…,n} without an odd number of consecutive integers is F(n+1)?

I have two questions about the Fibonacci sequence: I read from Wikipedia: 1) The number of subsets S ⊂ {1,...,n} without an odd number of consecutive integers is F(n+1). 2) The number of ...