Thursday, 30 November 2017

sequences and series - Relation between Fibonacci number and the golden section




We denote the nth term of Fibonacci number with Fn. Assume that
α=1+52. With simulation, I found the following
relation between Fibonacci number and the golden section
Fn+1Fnα1(Fn)2 .


Is there a analytical method that we can proof the mentioned formula. I would greatly appreciate for any suggestions.



Maple simulation




Edit: First I want to gratitude from Milo Brandt for nice answer. In continue, i want to generalize my question.



One of the most important generalization of the classical Fibonacci numbers is the Fibonacci p-step numbers that is defined as follows
F(p)n=F(p)n1+F(p)n2++F(p)np.


With boundary conditions
F(p)0=0,F(p)1=0,,F(p)p2=0,F(p)p1=1.



We can get the limit value of Fibonacci p-step numbers by inverse of solution of equation xp+12x+1=0 in the interval (0,1). We denote the limit value of Fibonacci p-step numbers with αp. In fact, αp is defined in the following form
αp=limnF(p)n+1F(p)n .


The generalization of the above formula is
F(p)n+1F(p)nαpF(p)n(pp1) .



For example for the case p=4, we have



Fibonacci $4$-step numbers


Answer



The most direct way to deal with the Fibonacci numbers is to use Binet's formula:
Fn=φn(φ)n5


where φ=1+52. So, the ratio of Fn+1Fn can be written in closed form as:
Fn+1Fn=φn+1(φ)n1φn(φ)n


Note that this obviously tends towards φ as n goes to , since the (φ)n terms quickly go to zero. Now, you additionally want to show that the difference between this ratio and φ shrinks with 1F2n. To do this, let us take that difference symbolically:
Fn+1Fnφ=φn+1(φ)n1φn(φ)nφn+1+(φ)n+1φn(φ)n=(φ)n+1(φ)n1φn(φ)n

Then, we use that (φ)1+φ=5 to simplify to
Fn+1Fnφ=5(φ)nφn(φ)n5(1)nφ2n(1)n5F2n

where the signs indicate that the ratio of the two sides of the "equation" tends to 1 as n goes to . We use that Fnφn5, which implies F2nφ2n5


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