Nice hypergeometric series involving the silver ratio δS
Today we show the proof of this result involving hypergeometric series and the silver ratio δS=1+√2 proposed by @mamekebi 3F2[1,1,1232,32,−1]=∫10arcsinh(x)x√1+x2dx=π28−ln2(1+√2)2 The proof relies on some properties of the rising factorial and some identities of the dilogarithm function.
Proof
Recall the recursion formula for the argument and the degree of the rising factorial: (x)n+1=(n+x)(x)n=x(x+1)n and the duplication formula (x)2n=22n(x2)n(x+12)n Hence 3F2[1,1,1232,32,−1]=∞∑j=0(1)j(1)j(12)j(32)j(32)j(−1)jj!=12∞∑j=0(1)j(1)j(32)j(j+12)(−1)jj!from (1)=∞∑j=0(1)j(1)j(32)j(2j+1)(−1)jj! We previously showed the proof of this series expansion: arcsinh(x)x√1+x2=∞∑j=022j(2jj)(2j+1)(−1)jx2j=∞∑j=022jj!2(2j)!(2j+1)(−1)jx2j=∞∑j=022j(1)j(1)j(1)2j(2j+1)(−1)jx2j((1)j=j!)=∞∑j=0(1)j(1)j(32)j(−1)jx2jj! from (1) and (2) Integrating from 0 to 1 ∫10arcsinh(x)x√1+x2dx=∞∑j=0(1)j(1)j(32)j(2j+1)(−1)jj!=3F2[1,1,1232,32,−1] I=∫10arcsinh(x)x√1+x2dx=∫10ln(√x2+1+x)x√1+x2dx=2∫1+√21ln(w)(w+1)(w−1)dw(w↦√x2+1+x)=−∫1+√21ln(w)1−wdw−∫1+√21ln(w)w+1dw=∫−√20ln(1−t)tdw−∫1+√21ln(w)w+1dw(t↦1−w)IBP=∫−√20ln(1−t)tdw−ln(1+√2)ln(2+√2)−∫−1−1−√2ln(1−w)wdw=−∫0−√2ln(1−t)tdw−ln(1+√2)ln(2+√2)−∫0−1−√2ln(1−w)wdw+∫0−1ln(1−w)wdw=−Li2(−√2)−ln(1+√2)ln(2+√2)−Li2(−1−√2)+Li2(−1) From the identity Li2(−z)−Li2(1−z)+12Li2(1−z2)=−π212−lnz⋅ln(z+1) If we put z=√2 Li2(−√2)=Li2(1−√2)−12Li2(−1)−π212−ln√2⋅ln(√2+1)(3) From the identity Li2(z)+Li2(1z)=−π26−ln2(−z)2 If we put z=1−√2
Therefore Li2(−1−√2)=−Li2(1−√2)−π26−ln2(√2−1)2(4) Using the fact that Li2(−1)=−π212 and from (3) and (4) we have: I=π28+ln√2⋅ln(√2+1)−ln(1+√2)ln(2+√2)+ln2(√2−1)2=π28−ln2(1+√2)2 3F2[1,1,1232,32,−1]=∫10arcsinh(x)x√1+x2dx=π28−ln2(1+√2)2