Forgotten functions used to calculate π
Today we will show the proof of the following result:
1+(12)2+(12⋅4)2+(1⋅32⋅4⋅6)2+(1⋅3⋅52⋅4⋅6⋅8)2+...=4π
We show the proof of this result using an elliptic integrals approach (the result can also be obtained trough the Vandermonde's identity). However, following the path of elliptic integrals turned out very interesting given that it needs a tour through some of the main results of the theory of this sepcial functions to prove it.
This is the outline of the proof:
- Obtain the series expansion for E(p),K(p) (the elliptic integrals of the first and second kind)
- Prove a result know as the Landen transformation
- Obtain the following series expansion as corollary of 1 and 2 (our result is a particular case of this expansion)
E(p)=π(1+q)4∞∑n=0[(2n−3)!!(2n)!!]2(1−q1+q)2n p=√1−q2
where E(p) is the elliptic integral of the second kind, p is known as the modulus and q is known as de complementary modulus. If E(p)=1 when p=1 we get our result.
Proof
Consider the complete elliptic integral of the first kind:
K(p)=∫π20dθ√1−p2sin2θ
where p is known as the modulus.
We will obtain its series expansion:
Proposition.
K(p)=π2∞∑j=0(2j−1)!!2(2j)!!2p2j Proof
First, recall that
arcsin(x)=∞∑j=0(2j−1)!!(2j)!!x2j+12j+1|x|≤1 Hence 1√1−x2=arcsin′(x)=∞∑j=0(2j−1)!!(2j)!!x2j|x|≤1 Now, given that −1<sinθ<1 then 0<p2sin2θ<p2
Therefore
If p2≤1 1√1−p2sin2θ=∞∑j=0(2j−1)!!(2j)!!p2jsin2jθ Integrating ∫π20dθ√1−p2sin2θ=∫π20∞∑j=0(2j−1)!!(2j)!!p2jsin2jθdθ=∞∑j=0(2j−1)!!(2j)!!p2j∫π20sin2jθdθ It is easy to show that (proof will be added in the Appendix) ∫π20sin2jθdθ=(2j−1)!!(2j)!!π2 Hence K(p)=∫π20dθ√1−p2sin2θ=π2∞∑j=0(2j−1)!!2(2j)!!2p2j
Now we are going to prove the expansion series for the complete elliptic integral of the second kind:
Proposition
E(p)=∫π20√1−p2sin2θdθ=π2∞∑j=011−2j[(2j−1)!!(2j)!!pj]2
Proof
Lets apply the Feynman's trick: ddpE(p)=−∫π20psin2θ√1−p2sin2θdθ Hence −psin2θ√1−p2sin2θ=−∞∑j=0(2j−1)!!(2j)!!p2j+1sin2j+2(θ) Therefore −∫π20psin2θ√1−p2sin2θdθ=−∫π20∞∑j=0(2j−1)!!(2j)!!p2j+1sin2j+2(θ)dθ=−∞∑j=0(2j−1)!!(2j)!!p2j+1∫π20sin2j+2(θ)dθ=−∞∑j=0π2(2j−1)!!(2j)!!p2j+1(2j+1)!!(2j+2)!!=−π2∞∑j=0(2j−1)!!2(2j+2)!!2(2j+1)(2j+2)p2j+1
where we make use of the fact that (the proof will be added to the appendix)
∫π20sin2j+2(θ)dθ=(2j+1)!!(2j+2)!! Hence, integrating with respect to p ∫π20√1−p2sin2θdθ=−π2∞∑j=0(2j−1)!!2(2j)!!2(2j+1)(2j+2)2p2j+2+C=−π2∞∑j=0(2j+1)!!2(2j+2)!!21(2j+1)p2j+2+C Now if p=0 then C=π2 therefore ∫π20√1−p2sin2θdθ=π2−π2∞∑j=0(2j+1)!!2(2j+2)!!21(2j+1)p2j+2 If m=j+1 ∫π20√1−p2sin2θdθ=π2−π2∞∑m=1(2m−1)!!2(2m)!!21(2m−1)p2m=π2∞∑m=0(2m−1)!!2(2m)!!21(1−2m)p2m Therefore E(p)=π2∞∑m=0(2m−1)!!2(2m)!!21(1−2m)p2m
Now we have to make use of the Landen transformation (the proof is very extensive and will be added in the appendix later)
Proposition (Landen transformation) K(1−q1+q)=1+q2K(p) E(1−q1+q)=E(p)+qK(p)1+q
Finally, we are going to show the proof for the following expansion of E(p)
Proposition E(p)=π(1+q)4∞∑n=0[(2n−3)!!(2n)!!]2(1−q1+q)2n
Proof
From (3) we can write: E(p)=E(1−q1+q)−qK(p) Substituting (4) in (3) we can write E(p)=E(1−q1+q)−qK(p)=E(1−q1+q)−2q1+qK(1−q1+q) Then, using the power series for E,K in (1) and (2): K(1−q1+q)=π2∞∑j=0(2j−1)!!2(2j)!!2(1−q1+q)2j E(1−q1+q)=π2∞∑m=0(1−2m)!!2(2m)!!21(1−2m)(1−q1+q)2m Hence E(p)=π(1+q)2∞∑n=011−2n[(2n−1)!!(2n)!!]2(1−q1+q)2n−2q1+qπ2∞∑n=0[(2n−1)!!(2n)!!]2(1−q1+q)2n=π(1+q)2∞∑n=0{[(2n−1)!!(2n)!!]2(1−q1+q)2n(11−2n−2q(1+q)2)}=π(1+q)4∞∑n=0[(2n−1)!!(2n)!!]2(1−q1+q)2n[21−2n−1+(1−q1+q)2]=π(1+q)4{1−∞∑n=1[(2n−1)!!(2n)!!]2(1−q1+q)2n2n+12n−1+∞∑n=0[(2n−1)!!(2n)!!]2(1−q1+q)2(n+1)}=π(1+q)4{1−∞∑n=1[(2n−1)!!(2n)!!]2(1−q1+q)2n2n+12n−1+∞∑n=1[(2n−3)!!(2n−2)!!]2(1−q1+q)2n}=π(1+q)4{1+∞∑n=1[(2n−3)!!(2n)!!]2(1−q1+q)2n[(2n)2−2n+12n−1(2n−1)2]}=π(1+q)4{1+∞∑n=1[(2n−3)!!(2n)!!]2(1−q1+q)2n}=π(1+q)4∞∑n=0[(2n−3)!!(2n)!!]2(1−q1+q)2n Hence E(p)=π(1+q)4∞∑n=0[(2n−3)!!(2n)!!]2(1−q1+q)2n Note that if p=1 then q=0. Moreover, E(1)=∫π20√1−sin2θdθ=∫π20cosθdθ=1 Therefore, we can conclude that 1+(12)2+(12⋅4)2+(1⋅32⋅4⋅6)2+(1⋅3⋅52⋅4⋅6⋅8)2+...=4π
Appendix I will be adding the omitted results the following days