Definite Integral

bincybn

Member
Dear All,

$$\displaystyle \int_{0}^{1}x*\left(\left(-ln(x)\right)^{k}\right)*\left(1-x\right)^{(N-1)}dx$$, where k is an odd no. N >=2.

regards,
Bincy

Sudharaka

Well-known member
MHB Math Helper
Dear All,

$$\displaystyle \int_{0}^{1}x*\left(\left(-ln(x)\right)^{k}\right)*\left(1-x\right)^{(N-1)}dx$$, where k is an odd no. N >=2.

regards,
Bincy

Hi bincybn,

Your integral cannot be expressed in terms of elementary functions. However using Maxima I found that the answer is,

$\displaystyle \int_{0}^{1}x((-ln(x))^{k}\left(1-x\right)^{(N-1)}dx=(-1)^k\left( \left. \frac{{d}^{k}}{d\,{x}^{k}}\,\beta\left(N,x\right) \right|_{x=2}\right)~~~~\mbox{for }k\in\mathbb{Z^+}$

$$\beta$$ is the Beta function.

• bincybn

dwsmith

Well-known member
Hi bincybn,

Your integral cannot be expressed in terms of elementary functions. However using Maxima I found that the answer is,

$\displaystyle \int_{0}^{1}x((-ln(x))^{k}\left(1-x\right)^{(N-1)}dx=(-1)^k\left( \left. \frac{{d}^{k}}{d\,{x}^{k}}\,\beta\left(N,x\right) \right|_{x=2}\right)~~~~\mbox{for }k\in\mathbb{Z^+}$

$$\beta$$ is the Beta function.
Since the integral is set up like the beta function, maybe he was supposed to obtain the solution by induction.

• Sudharaka

Sudharaka

Well-known member
MHB Math Helper
Since the integral is set up like the beta function, maybe he was supposed to obtain the solution by induction.
Hi dwsmith, Yes, maybe. But without knowing the answer it is impossible to use the induction method. Isn't?

Amer

Active member
Gamma function $$\Gamma (z) = \int_{0}^{\infty} e^{-t} t^{z-1} dt$$

$$\int_{0}^{1} x((- \ln x )^k) (1-x)^{n-1} dx$$

Let $$u = - \ln x \Rightarrow x = e^{-u} \Rightarrow dx = - e^{-u} du$$

$$x = 0 \rightarrow u = \infty , x = 1 \rightarrow u = 0$$

$$\int_{\infty}^{0} e^{-u} (u^k )(1-e^{-u})^n(-e^{-u}) du$$

$$\int_{0}^{\infty} e^{-2u} (u^k )(1-e^{-u})^n du$$

Note that
$$(1-e^{-u})^n = \sum_{i=0}^{n} \dbinom{n}{i} (-1)^i (e^{-u})^i$$

and $$\int_{0}^{\infty} e^{-at} t^{z-1} dt = \frac{\Gamma (z)}{a}$$

$$\int_{0}^{\infty} e^{-2u} (u^k )(1-e^{-u})^n du = \sum_{i=0}^{n} (-1)^i \int_{0}^{\infty} (u^k)(e^{-u(2+i)}) du$$

$$\sum_{i=0}^{n} (-1)^i \int_{0}^{\infty} (u^k)(e^{-u(2+i)}) du = \sum_{i=0}^{\infty}\frac{(-1)^i\Gamma (k+1)}{2+i}$$

I solved it for n instead of n-1

Last edited:
• bincybn

bincybn

Member
Thanks everyone.

To Amer: Thanks for ur method. But some mistakes are here and there like.
$$\int_{0}^{\infty} e^{-at} t^{z-1} dt = \frac{\Gamma (z)}{a}$$
which is in fact $$\int_{0}^{\infty} e^{-at} t^{z-1} dt = \frac{\Gamma (z)}{a^z}$$

regards,
Bincy