Mathematics Asked by Ali Shadhar on November 15, 2020
I am not sure if there exists a closed form for
$$I=int_0^{pi/2}frac{x^2 text{Li}_2(sin^2x)}{sin x}dx$$
which seems non-trivial.
I used the reflection and landen’s identity, didn’t help much.
In case you are curious how I came up with this integral:
From here we have
$$arcsin^3(x)=6sum_{k=1}^inftyleft[sum_{m=0}^{k-1}frac{1}{(2m-1)^2}right]frac{{2kchoose k}}{4^k}frac{x^{2k+1}}{2k+1},quad |x|<1$$
differentiate both sides with respect to $x$ we get
$$frac{arcsin^2(x)}{sqrt{1-x^2}}=2sum_{k=1}^inftyleft[sum_{m=0}^{k-1}frac{1}{(2m-1)^2}right]frac{{2kchoose k}}{4^k}x^{2k}$$
use $sum_{m=0}^{k-1}frac{1}{(2m-1)^2}=H_{2k}^{(2)}-frac14H_k^{(2)}$ and replace $x$ by $sqrt{x}$ we get the form
$$frac{arcsin^2(sqrt{x})}{sqrt{1-x}}=2sum_{k=1}^inftyleft[H_{2k}^{(2)}-frac14H_k^{(2)}right]frac{{2kchoose k}}{4^k}x^{k}$$
Divide both sides by $x$ then $int_0^y$ we have
$$int_0^yfrac{arcsin^2(sqrt{x})}{xsqrt{1-x}}dx=2sum_{k=1}^inftyleft[H_{2k}^{(2)}-frac14H_k^{(2)}right]frac{{2kchoose k}}{4^k}frac{y^{k}}{k}$$
Next, multiply both sides by $-frac{ln(1-y)}{y}$ then $int_0^1$ and use $-int_0^1 y^{k-1}ln(1-y)dy=frac{H_k}{k}$
$$2sum_{k=1}^inftyleft[H_{2k}^{(2)}-frac14H_k^{(2)}right]frac{{2kchoose k}}{4^k}frac{H_k}{k^2}=-int_0^1int_0^yfrac{arcsin^2(sqrt{x})ln(1-y)}{xysqrt{1-x}}dxdy$$
$$=int_0^1frac{arcsin^2(sqrt{x})}{xsqrt{1-x}}left(-int_x^1frac{ln(1-y)}{y}dyright)dx$$
$$=int_0^1frac{arcsin^2(sqrt{x})}{xsqrt{1-x}}left(zeta(2)-text{Li}_2(x)right)dx$$
$$overset{sqrt{x}=sintheta}{=}2int_0^{pi/2}frac{x^2}{sin x}(zeta(2)-text{Li}_2(sin^2x))dx$$
So we have
$$sum_{k=1}^inftyleft[H_{2k}^{(2)}-frac14H_k^{(2)}right]frac{{2kchoose k}}{4^k}frac{H_k}{k^2}=zeta(2)int_0^{pi/2}frac{x^2}{sin x}dx-int_0^{pi/2}frac{x^2 text{Li}_2(sin^2x)}{sin x}dx$$
The first integral can be calculated by applying integration by parts then using the fourier series of $ln(tanfrac x2)$.
Another question is, clearly the two sums on the LHS are convergent as the denominator blows to infinity much faster than the numerator. But does there exist a closed form for each?
All methods are welcome. Thank you
Get help from others!
Recent Questions
Recent Answers
© 2024 TransWikia.com. All rights reserved. Sites we Love: PCI Database, UKBizDB, Menu Kuliner, Sharing RPP