Series involving K(p) and the Catalan constant
Today we show the proof of this result posted by @seriesbot_q on Twitter: β(2)=π4∞∑n=0(2nn)224n(2n+1) This series turned out being a slight variation of the series expansion of the complete elliptic integral of the first kind K(p) .
Proof
Recall the definition of the complete elliptic integral of the first kind: K(p)=∫π201√1−p2sin2θdθ|p|<1
First, we are going to obtain its series expansion.
In a previous post we proved the following result: 1√1−x=∞∑n=0(2n−1)!!(2n)!!xn0≤x<1 We know that −1≤sinθ≤1 |p|<1 ⟹p2sin2θ<1 Hence 1√1−p2sin2θ=∞∑n=0(2n−1)!!(2n)!!p2nsin2n(θ) Integrating from 0 to π2 K(p)=∫π201√1−p2sin2θdθ=∫π20∞∑n=0p2n(2n−1)!!(2n)!!sin2n(θ)dθ=∞∑n=0(2n−1)!!(2n)!!p2n∫π20sin2n(θ)dθ=∞∑n=0(2n−1)!!(2n)!!p2n∫10w2n(1−w2)−12dw(w↦sinθ)=12∞∑n=0(2n−1)!!(2n)!!p2n∫10sn+12−1(1−s)12−1ds(s↦w2)=12∞∑n=0(2n−1)!!(2n)!!p2nB(n+12,12)=12∞∑n=0(2n−1)!!(2n)!!p2nΓ(n+12)Γ(12)Γ(n+1) From the properties of the double factorial we have: (2n−1)!!=2nΓ(n+12)√π (2n)!!=2nn!=2nΓ(n+1) Hence K(p)=12∞∑n=0(2n−1)!!(2n)!!Γ(n+12)Γ(12)Γ(n+1)p2n=π2∞∑n=0(2n−1)!!2(2n)!!2p2n Recall the we can also write (2n−1)!!(2n)!!=(2nn)22n Therefore, this is the expansion series of the complete elliptic integral of the first kind: K(p)=π2∞∑n=0(2n−1)!!2(2n)!!2p2n=π2∞∑n=0(2nn)24n2p2n Now, integrating from 0 to 1 ∫10K(p)dp=π2∞∑n=0(2nn)224n(2n+1) The right hand side is the series that we are looking for. Then, we just have to find the solution to the integral in the left hand side: ∫10K(p)dp=∫10∫π201√1−p2sin2θdθdp=∫π20∫101√1−p2sin2θdpdθ=∫π20∫sinθ01√1−w2sinθdwdθ(w↦psinθ)=∫π201sinθ∫sinθ01√1−w2dwdθ=∫π201sinθarcsin(sinθ)dθ=∫π20θsinθdθ We solved this last integral in another post and the solution turned out being 2β(2). Hence ∫10K(p)dp=2β(2) Therefore β(2)=π4∞∑n=0(2nn)224n(2n+1)