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+1Fn−α∣≈1(Fn)2 .
Is there a analytical method that we can proof the mentioned formula. I would greatly appreciate for any suggestions.
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)n−1+F(p)n−2+⋯+F(p)n−p.
With boundary conditions
F(p)0=0,F(p)1=0,⋯,F(p)p−2=0,F(p)p−1=1.
We can get the limit value of Fibonacci p-step numbers by inverse of solution of equation xp+1−2x+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=limn→∞F(p)n+1F(p)n .
The generalization of the above formula is
∣F(p)n+1F(p)n−αp∣≈F(p)n−(pp−1) .
For example for the case p=4, we have
Answer
The most direct way to deal with the Fibonacci numbers is to use Binet's formula:
Fn=φn−(−φ)−n√5
where φ=1+√52. So, the ratio of Fn+1Fn can be written in closed form as:
Fn+1Fn=φn+1−(−φ)−n−1φn−(−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+1Fn−φ=φn+1−(−φ)−n−1φn−(−φ)−n−φn+1+(−φ)−n+1φn−(−φ)−n=−(−φ)−n+1−(−φ)−n−1φn−(−φ)−n
Then, we use that (φ)−1+φ=√5 to simplify to
Fn+1Fn−φ=√5(−φ)−nφn−(−φ)−n≈√5(−1)nφ2n≈(−1)n√5F2n
where the ≈ signs indicate that the ratio of the two sides of the "equation" tends to 1 as n goes to ∞. We use that Fn≈φn√5, which implies F2n≈φ2n5
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