Nice hypergeometric series involving the Fibonacci numbers F2k+3
Today we show the proof of this nice series proposed by @diegorattaggi ∞∑n=1(n+k−1k)Fn2n=2kF2k+3 For the proof we will use some properties of the rising factorial (Pochhammer polynomial), the Gaussian hypergeometric function and the Binet's formula for Fibonacci numbers.
Proof:
S=∞∑n=1(n+k−1k)Fn2n=∞∑n=1(n+k−1)!(n−1)!k!Fn2n=∞∑n=1Γ(n+k)(n−1)!k!Fn2n Recall that the pochhammer polynomial (rising factorial) can be expressed as the quotient of two gamma functions: Γ(n+x)Γ(x)=(x)n where (x)n=x(x+1)(x+2)⋯(x+n−1) Hence S=∞∑n=1Γ(n+k)(n−1)!k!Fn2n=Γ(k)k!∞∑n=1(k)n(n−1)!Fn2n=1k∞∑n=1(k)n(n−1)!Fn2n If we let the change of variable n=j+1 S=12k∞∑j=0(k)j+1j!Fj+12j From the recursion formula for the degree of the Pochhammer polynomial: (x)n+1=x(x+1)n we have S=12k∞∑j=0(k)j+1j!Fj+12j=12∞∑j=0(k+1)jj!Fj+12j From the Binet's formula Fj+1=ϕj+1−ψj+1√5 S=ϕ2√5∞∑j=0(k+1)jj!ϕj2j−ψ2√5∞∑j=0(k+1)jj!ψj2j Multiplying and dividing by (1)j we have an hypergeometric representation: S=ϕ2√5∞∑j=0(k+1)jj!ϕj2j−ψ2√5∞∑j=0(k+1)jj!ψj2j=ϕ2√5∞∑j=0(k+1)j(1)j(1)j(ϕ2)jj!−ψ2√5∞∑j=0(k+1)j(1)j(1)j(ψ2)jj!=ϕ2√5F(1,k+1;1;ϕ2)−ψ2√5F(1,k+1;1;ψ2) This is a special case of the Gaussian hypergeometric fucntion (generalized Binomial theorem): F(a,b;a;z)=(1−z)−b If we put a=1,b=k+1 S=ϕ2√5F(1,k+1;1;ϕ2)−ψ2√5F(1,k+1;1;ψ2)=ϕ2√5(1−ϕ2)−k−1−ψ2√5(1−ψ2)−k−1 Note ϕ2√5(1−ϕ2)−k−1=ϕ2√5(22−ϕ)k+1=ϕ2√5(3+√5)k+1=ϕ2√5(2+2ϕ)k+1=ϕ2k+12√5(1+ϕ)k+1=ϕ2k+12√5(ϕ2)k+1(1+ϕ=ϕ2)=2kϕ2k+3√5 ψ2√5(1−ψ2)−k−1=ψ2√5(1−1−ϕ2)−k−1(ψ=1−ϕ)=ψ2√5(21+ϕ)k+1=2kψ√5(1ϕ2)k+1(ϕ+1=ϕ2)=2kψ√5(ψ2)k+1=2kψ2k+3√5 Hence S=2kϕ2k+3√5−2k√5ψ2k+3=2k[ϕ2k+3√5−ψ2k+3√5]=2kF2k+3 Therefore ∞∑n=1(n+k−1k)Fn2n=2kF2k+3
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