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Thursday, February 17, 2022

Golden ratio II

An integral involving the Dirichlet eta function

Nice hypergeometric series involving the Fibonacci numbers F2k+3


Today we show the proof of this nice series proposed by @diegorattaggi n=1(n+k1k)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+k1k)Fn2n=n=1(n+k1)!(n1)!k!Fn2n=n=1Γ(n+k)(n1)!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+n1) Hence S=n=1Γ(n+k)(n1)!k!Fn2n=Γ(k)k!n=1(k)n(n1)!Fn2n=1kn=1(k)n(n1)!Fn2n If we let the change of variable n=j+1 S=12kj=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=12kj=0(k)j+1j!Fj+12j=12j=0(k+1)jj!Fj+12j From the Binet's formula Fj+1=ϕj+1ψj+15 S=ϕ25j=0(k+1)jj!ϕj2jψ25j=0(k+1)jj!ψj2j Multiplying and dividing by (1)j we have an hypergeometric representation: S=ϕ25j=0(k+1)jj!ϕj2jψ25j=0(k+1)jj!ψj2j=ϕ25j=0(k+1)j(1)j(1)j(ϕ2)jj!ψ25j=0(k+1)j(1)j(1)j(ψ2)jj!=ϕ25F(1,k+1;1;ϕ2)ψ25F(1,k+1;1;ψ2) This is a special case of the Gaussian hypergeometric fucntion (generalized Binomial theorem): F(a,b;a;z)=(1z)b If we put a=1,b=k+1 S=ϕ25F(1,k+1;1;ϕ2)ψ25F(1,k+1;1;ψ2)=ϕ25(1ϕ2)k1ψ25(1ψ2)k1 Note ϕ25(1ϕ2)k1=ϕ25(22ϕ)k+1=ϕ25(3+5)k+1=ϕ25(2+2ϕ)k+1=ϕ2k+125(1+ϕ)k+1=ϕ2k+125(ϕ2)k+1(1+ϕ=ϕ2)=2kϕ2k+35 ψ25(1ψ2)k1=ψ25(11ϕ2)k1(ψ=1ϕ)=ψ25(21+ϕ)k+1=2kψ5(1ϕ2)k+1(ϕ+1=ϕ2)=2kψ5(ψ2)k+1=2kψ2k+35 Hence S=2kϕ2k+352k5ψ2k+3=2k[ϕ2k+35ψ2k+35]=2kF2k+3 Therefore n=1(n+k1k)Fn2n=2kF2k+3

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