Another Headache for you

Calculus Level 5

0 1 ( ln x ) 4 ( 1 x ) 4 d x = A ζ ( P ) + B ζ ( Q ) + C ζ ( R ) \large \int_{0}^{1} \dfrac{(\ln x)^4}{(1-x)^4}\, dx = A\zeta(P)+B\zeta(Q)+C\zeta(R)

If the equation above is true for positive integers A A , B B , C C , P P , Q Q , and R R , find A + B + C + P + Q + R A+B+C+P+Q+R .

Notation : ζ ( ) \zeta(\cdot) denotes the Riemann zeta function .


The answer is 33.

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2 solutions

If J ( a ) = 0 1 x a d x = 1 a + 1 \displaystyle J(a)=\int_{0}^{1} x^a dx=\frac{1}{a+1} , then J ( b ) ( a ) = 0 1 x a ( ln x ) b d x = ( 1 ) b b ! ( a + 1 ) b + 1 \displaystyle J^{(b)}(a) = \int_{0}^{1}x^a (\ln x)^b dx= \frac{(-1)^b b!}{(a+1)^{b+1}}

S = 0 1 ( ln x ) 4 ( 1 x ) 4 d x = m = 1 ( m + 2 3 ) 0 1 x m ln 4 x d x = m = 1 ( m + 2 3 ) J ( 4 ) ( m ) \displaystyle S=\int_{0}^{1} \frac{(\ln x)^4}{(1-x)^4} dx = \sum_{m=1}^{\infty} \binom{m+2}{3} \int_{0}^{1} x^m \ln^4 x dx = \sum_{m=1}^{\infty} \binom{m+2}{3} J^{(4)}(m)

So , S = m = 1 ( m + 2 ) ! 3 ! ( m 1 ) ! ( 1 ) 4 4 ! m 5 \displaystyle S=\sum_{m=1}^{\infty} \frac{(m+2)!}{3!(m-1)!} \frac{(-1)^4 4!}{m^5}

S = m = 1 4 ( m + 1 ) ( m + 2 ) m 4 = m = 1 4 m 2 + 12 m 3 + 8 m 4 = 4 ζ ( 2 ) + 12 ζ ( 3 ) + 8 ζ ( 4 ) \displaystyle S = \sum_{m=1}^{\infty} 4 \frac{(m+1)(m+2)}{m^4} = \sum_{m=1}^{\infty} \frac{4}{m^2}+\frac{12}{m^3}+\frac{8}{m^4} = 4\zeta(2)+12\zeta(3)+8\zeta(4)

Incredible Mind
Jun 12, 2016

Pretty simple.Just expand (1-x)^(-4) in series form then use gamma function definition

Can you elaborate on it?

Pi Han Goh - 5 years ago

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