Monday, October 7, 2019

Inequality for Fibonacci sequence



Let {fn}(n1) Fibonacci sequence, which satisfies the following recurrence relation;
f1=f2=1,fn+2=fn+fn+1a(n1)
A few months ago I found an interesting inequality;
NN;aNn=1fnfn+1>1+52N(1)
This can be proved amazingly simple;

Let SN=Nn=1fnfn+1 and TN=Nn=1fn+1fn. By Cauchy-Schwarz inequality SNTN>N2. Using the given recurrence relation we can easily prove that $T_NT_N S_N>N^2,soS_N^2+NS_N-N^2>0.Solvingthisquadraticinequality,weget\displaystyle S_N>\frac{-1+\sqrt{5}}{2}N$.

After proving (1) I tried to find the upper bound of the sum by using similar method. I finally found
NN;aNn=1fn+1fn<1+52N+Nn=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<512+1n+1


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