Deal with odd $m$ in integral $\int_{0}^{\frac{\pi}{4}}(\sin^{6}m x+\cos^{6}m x) \ln (1+\tan x) d x $

calculusintegrationreal-analysistrigonometry

Latest edit

  1. Thanks to @Quanto for settling down the question by proving the odd one as:$$I_{2n+1}= \frac{5\pi}{64}\ln2+\frac3{16(2n+1)}\bigg(\frac\pi4-\sum_{j=0}^{2n}\frac{(-1)^j}{2j+1} \bigg)$$
    By our results for both odd and even multiples $n$ of $x$, we can conclude that

$$
\lim _{n \rightarrow \infty} \displaystyle \int_{0}^{\frac{\pi}{4}}\left[\sin ^{6}(nx)+\cos ^{6}(nx)\right] \ln (1+\tan x) d x =\frac{5 \pi\ln 2}{64}
$$

  1. As asked by @Claude Leibovici for the powers other than 6, I had generalised my result to even powers below as an answer:
    $$
    I(m,n):=\int_{0}^{\frac{\pi}{4}}\left[\cos ^{2 m}(2 nx)+\sin ^{2 m}(2 n x)\right] \ln (1+\tan x) d x= \frac{\pi \ln 2}{4} \cdot \frac{(2 m-1) ! !}{(2 m) ! !}
    $$

In order to evaluate the even case

$$\int_{0}^{\frac{\pi}{4}}\left[\sin^{6}(2 n x)+\cos^{6}(2 nx)\right] \ln (1+\tan x) d x $$
we first simplify
$\displaystyle \begin{aligned}\sin ^{6}(2 n x)+\cos ^{6}(2 n x) =& {\left[\sin ^{2}(2 n x)+\cos ^{2}(2 n x)\right]\left[\sin ^{4}(2 n x)-\sin ^{2}(2 n x) \cos ^{2}(2 n x)\right) } \\&\left.+\cos ^{4}(2 n x)\right] \\=& 1-3 \sin ^{2}(2 n x) \cos ^{2}(2 n x) \\=& 1-\frac{3}{4} \sin ^{2}(4 n x) \\=& 1-\frac{3}{8}(1-\cos 8 n x) \\=& \frac{1}{8}(5+3 \cos (8nx))\end{aligned} \tag*{} $

To get rid of the natural logarithm, a simple substitution transforms the integral into

$\begin{aligned}I &=\frac{1}{8} \int_{0}^{\frac{\pi}{4}}(5+3 \cos (8 n x)) \ln (1+\tan x) d x \\& \stackrel{x\mapsto\frac{\pi}{4}-x}{=} \frac{1}{8} \int_{0}^{\frac{\pi}{4}}(5+3 \cos (8 n x)) \ln \left(1+\tan \left(\frac{\pi}{4}-x\right)\right) d x \\&=\frac{1}{8} \int_{0}^{\frac{\pi}{4}}(5+3 \cos (8 n x)) \ln \left(\frac{2}{1+\tan x}\right) d x \\&=\frac{1}{8} \ln 2 \int_{0}^{\frac{\pi}{4}}(5+3 \cos (8 n x) )d x-I \\I &=\frac{\ln 2}{16} \int_{0}^{\frac{\pi}{4}}(5+3 \cos 8 n x) d x\\&=\frac{\ln 2}{16}\left[5 x+\frac{3}{8 n} \sin (8 n x)\right]_0^{\frac{\pi}{4} }\\ &=\frac{5 \pi}{64} \ln 2\end{aligned} \tag*{} $

My Question:

How can we deal with the odd one
$$\displaystyle \int_{0}^{\frac{\pi}{4}}\left[\sin ^{6}(2 n +1)x+\cos ^{6}(2 n +1)x\right] \ln (1+\tan x) d x ?$$

Can you help?

Best Answer

Similar to the even case, recognize $$\sin ^{6}mx+\cos ^{6}mx = \frac58+\frac38 \cos4mx $$ and rewrite the integral as follows \begin{align} I_{2n+1}=&\int_{0}^{\frac{\pi}{4}}\left[\sin ^{6}(2 n +1)x+\cos ^{6}(2 n +1)x\right] \ln (1+\tan x) d x \\ =&\> \frac58\int_{0}^{\frac{\pi}{4}}\ln \overset{\frac\pi4-x \to x}{(1+\tan x) } d x + \frac38\int_{0}^{\frac{\pi}{4}} \cos4(2n+1)x\ln\frac{\sqrt2\cos(\overset{\frac\pi4-x \to x}{\frac\pi4-x})}{\cos x}dx\\ =&\> \frac58\cdot \frac\pi8\ln2- \frac34\int_{0}^{\frac{\pi}{4}} \cos4(2n+1)x\ln \cos x\> \overset{ibp}{dx}\\ =&\> \frac{5\pi}{64}\ln2-\frac3{16(2n+1)} \int_{0}^{\frac{\pi}{4}}\frac{\sin 4(2n+1)x\sin x}{\cos x}dx\tag1 \end{align} Derive the integral below recursively \begin{align} K_m=&\int_{0}^{\frac{\pi}{4}}\frac{\sin 4m x\sin x}{\cos x}dx = K_{m-1}+\frac{(-1)^{m-1}}{2m-1}=-\frac\pi4+\sum_{j=0}^{m-1} \frac{(-1)^j}{2j+1}\\ \end{align} Then, evaluate $K_{2n+1}$ and plug into (1) to obtain $$I_{2n+1}= \frac{5\pi}{64}\ln2+\frac3{16(2n+1)}\bigg(\frac\pi4-\sum_{j=0}^{2n}\frac{(-1)^j}{2j+1} \bigg) $$ In contrast, the even case $I_{2n}= \frac{5\pi}{64}\ln2$ is much simpler.

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