Let {fn}(n≥1) Fibonacci sequence, which satisfies the following recurrence relation;
f1=f2=1,fn+2=fn+fn+1a(n≥1)
A few months ago I found an interesting inequality;
∀N∈N;aN∑n=1fnfn+1>−1+√52N⋯(1)
This can be proved amazingly simple;
Let SN=N∑n=1fnfn+1 and TN=N∑n=1fn+1fn. By Cauchy-Schwarz inequality SNTN>N2. Using the given recurrence relation we can easily prove that $T_N
After proving (1) I tried to find the upper bound of the sum by using similar method. I finally found
∀N∈N;aN∑n=1fn+1fn<1+√52N+N∑n=11n⋯(2)
However, my proof of (2) is long and complex than I expected; Is there any method to obtain (better) upper bound by using simple method like the proof of (1)? Let's share some ideas.
Answer
HINT A stronger statement is in fact true. fn+1fn<1+√52+1n
and
fnfn+1<√5−12+1n+1
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