Too many parts, part 2 [pun intended] (My fifteenth integral problem)

Calculus Level 5

0 1 x 2 2 x ln x d x \large \displaystyle \int_{0}^{1} x^{2} 2^{x} \ln x \, dx

If the above integral can be expressed in the highly unusual form

a li ( b ) + c + d γ + ln ( ln f ( g ) h ) ln j ( k ) \dfrac{-a \, \text{li}(b) + c +d \gamma + \ln \left( \dfrac{\ln^{f} (g)}{h} \right)}{\ln^{j}(k)}

where a , b , c , d , f , g , h , j , k a,b,c,d,f,g,h,j,k are positive integers and the above expression is in simplest form, find a + b + c + d + f + g + h + j + k a+b+c+d+f+g+h+j+k .

Notations :

  • li ( x ) \text{li}(x) is the logarithmic integral function. li ( x ) = 0 x d t ln ( t ) \text{li}(x) = \displaystyle \int_{0}^{x} \dfrac{dt}{\ln(t)}
  • γ \gamma is the Euler-Mascheroni constant, which may be represented as either lim n ( ln ( n ) + k = 1 n 1 k ) \displaystyle \lim_{n \to \infty} \left( -\ln(n) + \sum_{k=1}^{n} \dfrac{1}{k} \right) or 1 ( 1 x 1 x ) d x \displaystyle \int_{1}^{\infty} \left( \dfrac{1}{\left \lfloor x \right \rfloor} - \dfrac{1}{x}\right)\, dx


The answer is 22.

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1 solution

Mark Hennings
Feb 23, 2016

After determined integration by parts, we have x 2 e a x ln x d x = ( 1 a x 2 2 a 2 x ) e a x ln x + 2 a 2 ( e a x 1 ) ln x ( 1 a 2 3 a 3 ) e a x 2 a 3 e a x 1 x d x \int x^2 e^{ax} \ln x\,dx \; = \; \big(\tfrac1a x^2 - \tfrac2{a^2} x\big)e^{ax}\ln x + \tfrac{2}{a^2}(e^{ax} - 1)\ln x - \big(\tfrac{1}{a^2} - \tfrac{3}{a^3}\big)e^{ax} - \tfrac{2}{a^3} \int \frac{e^{ax}-1}{x}\,dx and hence 0 1 x 2 e a x ln x d x = ( 1 a 2 3 a 3 ) e a 3 a 3 2 a 3 0 1 e a x 1 x d x = ( 1 a 2 3 a 3 ) e a 3 a 3 2 a 3 0 a e x 1 x d x = ( 1 a 2 3 a 3 ) e a 3 a 3 2 a 3 [ l i ( e a ) ln a γ ] = 2 l i ( e a ) + 3 ( e a 1 ) + 2 γ + ln a 2 a e a a 3 \begin{array}{rcl} \displaystyle \int_0^1 x^2 e^{ax} \ln x\,dx & = & \displaystyle -\big(\tfrac{1}{a^2} - \tfrac{3}{a^3}\big)e^a - \tfrac{3}{a^3} - \tfrac{2}{a^3}\int_0^1 \frac{e^{ax}-1}{x}\,dx \\ & = & \displaystyle -\big(\tfrac{1}{a^2} - \tfrac{3}{a^3}\big)e^a - \tfrac{3}{a^3} - \tfrac{2}{a^3}\int_0^a \frac{e^x-1}{x}\,dx \\ & = & \displaystyle -\big(\tfrac{1}{a^2} - \tfrac{3}{a^3}\big)e^a - \tfrac{3}{a^3} - \tfrac{2}{a^3}\big[\mathrm{li}(e^a) - \ln a - \gamma\big] \\ & = & \displaystyle \frac{-2\mathrm{li}(e^a) + 3(e^a-1) + 2\gamma + \ln a^2 - ae^a}{a^3} \end{array} for any a > 0 a > 0 . Putting a = ln 2 a = \ln2 , we obtain 0 1 x 2 2 x ln x d x = 2 l i ( 2 ) + 3 + 2 γ + ln ( 1 4 ( ln 2 ) 2 ) ( ln 2 ) 3 \begin{array}{rcl} \displaystyle \int_0^1 x^2 2^x \ln x\,dx & = &\displaystyle \frac{-2\mathrm{li}(2) + 3 + 2\gamma + \ln\big(\frac14(\ln2)^2\big)}{(\ln 2)^3} \end{array} making the answer 2 + 2 + 3 + 2 + 2 + 2 + 4 + 3 + 2 = 22 2 + 2 + 3 + 2 + 2 + 2 + 4 + 3 + 2 \,=\, \boxed{22} .

Alternatively, with less integration by parts, we could evaluate 0 1 e a x ln x d x = γ + ln a l i ( e a ) a \int_0^1 e^{ax} \ln x\,dx \; = \; \frac{\gamma + \ln a - \mathrm{li}(e^a)}{a} and differentiate this twice with respect to a a .

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