Return to Revisions

Let $Q_{k}(x)=\sum_{k=0}^{2k}\binom{4k+1}{2j}(1-x)^{j}x^{2k-j}$. One can check that $Q_{0}(x)=1, Q_{1}(x)=5-4x^2$ and for $k\geq 2$ we have the following recurrence: $$ Q_{k}(x)=-2(4x^2-4x+4-1)Q_{k-1}(x)-(2x-1)^{4}Q_{k-2}(x). $$ Using standard methods one can find exact expression for $Q_{k}(x)$ in the form $Q_{k}(x)=P_{1}(x)r_{1}(x)^{k}+P_{2}(x)r_{2}(x)^{k}$, where $$ r_{1}(x)=1+4x-4 x^2-4 \sqrt{x-x^2},\quad r_{2}(x)=1+4x-4 x^2+4 \sqrt{x-x^2} $$ and $$ P_{1}(x)=\frac{x-1+\sqrt{(1-x)x}}{2 \sqrt{(1-x) x}},\quad P_{2}(x)=\frac{1-x+\sqrt{(1-x) x}}{2\sqrt{(1-x)x}}. $$ Using the identity $$ \sum_{k=0}^{\infty}\frac{u^{k}}{(2k+1)v^{4k+1}}=\frac{v \tanh ^{-1}\left(\frac{\sqrt{u}}{v^2}\right)}{\sqrt{u}} $$ (with appropriate values of $u, v$) one can obtain closed form expression for LHS (quite complicated) and RHS . However, if my calculations are not wrong, as stated, the formula is not correct. Take $x=3$. Then RHS=$\frac{\pi}{12\sqrt{3}}\approx 0.1511499470$ but $LHS\approx 0.1967733546$ (the values computed in Mathematica 12).