Bertrand's postulate says: For every n>1 there is always at least one prime p such that n<p<2n. Is the following statement: For every n>3 there is always at least one prime p such that F_n<p<F_(n+1) (F_n is n-th Fibonacci number). also valid? If it is invalid, is there a finite or infinite number of ns such that there is no prime between F_n and F_(n+1_?

vroos5p

vroos5p

Open question

2022-08-17

Bertrand's postulate says:
For every n > 1 there is always at least one prime p such that n < p < 2 n.
Is the following statement:
For every n > 3 there is always at least one prime p such that F n < p < F n + 1 ( F n is n-th Fibonacci number).
also valid?
If it is invalid, is there a finite or infinite number of ns such that there is no prime between F n and F n + 1 ?

This question is inspiblack by another question. I feel intuitively that it may be interesting, but don't have enough number theory background to tackle it.

Answer & Explanation

Bridget Vang

Bridget Vang

Beginner2022-08-18Added 11 answers

The conjecture is true.

For n 25, there is always a prime between n and 1.2 n. Note that F 9 = 34, F 10 = 55, and their ratio is 1.617 > 1.2. In fact, for all n 9, we can prove that F n + 1 F n > 1.2 (*). Hence, combining these results gives a prime between F n and F n + 1 for all n 9. It remains merely to check the smaller Fibonacci numbers.

(*) The ratio approaches ϕ = 1 + 5 2 1.6 as a limit, and rather quickly.

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