⎝ ⎛ n = 1 ∑ ∞ F n φ ( n ) ( 1 − ϕ ) n ⎠ ⎞ − 2 = ?
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This is quite nice, pleasantly elegant.
@Jake Lai Your questions have became a research project.
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Go read Apostol's Analytic Number Theory. PDFs can be found online. It'll give you a lot of inspiration.
Did the exact same! @Jake Lai Love your problems!
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For this we need to know two things:
‣ Lambert series
‣ Binets formula
Using binets formula we know F n = 5 ϕ n − ( 1 − ϕ ) n
S = n = 1 ∑ ∞ F n φ ( n ) ( 1 − ϕ ) n = 5 n = 1 ∑ ∞ ϕ n − ( 1 − ϕ ) n φ ( n ) ( 1 − ϕ ) n = 5 n = 1 ∑ ∞ 1 − ( ϕ 1 − ϕ ) n φ ( n ) ( ϕ 1 − ϕ ) n
Now we know that the lambert series for φ ( x ) where ∣ q ∣ < 1 is n = 1 ∑ ∞ 1 − q n φ ( n ) q n = ( 1 − q ) 2 q
Note ∣ ϕ 1 − ϕ ∣ < 1
∴ S = 5 ( 1 − ϕ 1 − ϕ ) 2 ϕ 1 − ϕ = 5 4 ϕ 2 − 4 ϕ + 1 ϕ ( 1 − ϕ ) = − 5 5
We can then evaluate the answer to be ( − 5 5 ) − 2 = 5