Not A Telescoping Sum

Calculus Level 5

k = 1 ( 1 3 k 1 1 3 k ) \large \sum_{k = 1}^{\infty} \left(\dfrac{1}{3k-1}-\dfrac{1}{3k}\right)

The above sum has a closed form of ln A B π C D 2 E \large \dfrac{\ln{A}}{B} - \dfrac{\pi\sqrt{C}}{D^2 E} \; where A , B , C , D A,B,C,D and E E are positive prime numbers. Find the value of A + B + C + D + E A+B+C+D+E .


The answer is 13.

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3 solutions

Akshat Sharma
Mar 25, 2016

k = 1 ( 1 3 k 1 1 3 k ) = k = 1 0 1 ( x 3 k 2 x 3 k 1 ) d x = 0 1 k = 1 ( x 3 k 2 x 3 k 1 ) d x = 0 1 x x 2 1 x 3 d x = l n ( 3 ) 2 3 Π ( 3 2 ) 2 \sum _{ k=1 }^{ \infty }{ \left( \frac { 1 }{ 3k-1 } -\frac { 1 }{ 3k } \right) } =\sum _{ k=1 }^{ \infty }{ \int _{ 0 }^{ 1 }{ { (x }^{ 3k-2 }-{ x }^{ 3k-1 }) } dx } \\ =\int _{ 0 }^{ 1 }{ \sum _{ k=1 }^{ \infty }{ { (x }^{ 3k-2 }-{ x }^{ 3k-1 } } )dx } \\ =\quad \int _{ 0 }^{ 1 }{ \frac { x-{ x }^{ 2 } }{ 1-{ x }^{ 3 } } dx } \quad \\ =\quad \frac { ln(3) }{ 2 } - \frac { \sqrt { 3 } \Pi }{ \left( { 3 }^{ 2 } \right) 2 }

Good approach

Chew-Seong Cheong - 5 years, 2 months ago

Interesting idea! Very simple and elegant solution.

Ariel Gershon - 5 years, 2 months ago

same method! :)

Hamza A - 5 years, 2 months ago
Chew-Seong Cheong
Mar 25, 2016

S = k = 1 ( 1 3 k 1 1 3 k ) = k = 1 2 sin ( ( k 1 ) 2 π 3 ) 3 k See Note. = [ 2 e i 2 π 3 3 k = 1 e i 2 k π 3 k ] = [ 2 e i 2 π 3 3 ln ( 1 e i 2 π 3 ) ] = [ 2 3 ( 1 2 i 3 2 ) ln ( 3 2 i 3 2 ) ] = [ ( 1 3 + i ) ln ( 3 ( 3 2 i 1 2 ) ) ] = [ ( 1 3 + i ) ln ( 3 e i π 6 ) ] = [ ( 1 3 + i ) ( ln 3 2 i π 6 ) ] = ln 3 2 3 π 18 \begin{aligned} S &= \sum_{k=1}^\infty \left( \frac{1} {3k-1}- \frac{1} {3k} \right) \\ & = \sum_{k=1}^\infty \frac{2\sin \left( (k-1)\frac {2 \pi}{3}\right)} {\sqrt{3} k} \quad \quad \small \color{#3D99F6}{\text{See Note.}} \\ & = \Im \left[ \frac {2 e^{-i\frac {2\pi} {3}}} {\sqrt {3} } \sum_{k=1}^\infty \frac {e^{i\frac {2 k \pi} {3}}}{k} \right] \\ & = \Im \left[ - \frac {2 e^{-i\frac {2 \pi} {3}}} {\sqrt {3} } \ln (1 - e^{i\frac { 2 \pi} {3}}) \right] \\ & = \Im \left[ - \frac {2} {\sqrt {3} } \left(-\frac {1} {2} - i\frac {\sqrt {3}} {2} \right) \ln \left(\frac {3} {2} - i\frac {\sqrt {3}} {2} \right) \right] \\ & = \Im \left[ \left(\frac {1} {\sqrt{3}} + i \right) \ln \left(\sqrt{3} \left(\frac {\sqrt {3}} {2} - i\frac {1} {2} \right)\right) \right] \\ & = \Im \left[ \left(\frac {1} {\sqrt{3}} + i \right) \ln \left(\sqrt{3} e^{-i\frac {\pi}{6}} \right) \right] \\ & = \Im \left[ \left(\frac {1} {\sqrt{3}} + i \right) \left(\frac {\ln 3}{2} -i\frac {\pi}{6} \right) \right] \\ & = \frac {\ln 3} {2} - \frac{\sqrt {3} \pi} {18} \end{aligned}

a + b + c + d + e = 3 + 2 + 3 + 3 + 2 = 13 \Rightarrow a+b+c+d+e=3+2+3+3+2=\boxed {13}


Note:

S = k = 1 ( 1 3 k 1 1 3 k ) = 1 2 1 3 + 1 5 1 6 + 1 8 1 9 + . . . = 0 1 + 1 2 1 3 + 0 4 + 1 5 1 6 + 0 7 + 1 8 1 9 + . . . = 2 3 ( 0 1 1 + 3 2 1 2 3 2 1 3 + 0 1 4 + 3 2 1 5 3 2 1 6 + 0 1 7 + 3 2 1 8 3 2 1 9 + . . . ) = 2 3 ( sin 0 1 1 + sin 2 π 3 1 2 sin 4 π 3 1 3 + sin 2 π 1 4 + sin 8 π 3 1 5 sin 10 π 3 1 6 + sin 4 π 1 7 + sin 14 π 3 1 8 sin 16 π 3 1 9 + . . . ) = k = 1 2 sin ( ( k 1 ) 2 π 3 ) 3 k \begin{aligned} S & = \sum_{k=1}^\infty \left( \frac{1} {3k-1}- \frac{1} {3k} \right) \\ & = \frac 12 - \frac 13 + \frac 15 - \frac 16 + \frac 18 - \frac 19 + ... \\ & = \frac 01 + \frac 12 - \frac 13 + \frac 04 + \frac 15 - \frac 16 + \frac 0 7 + \frac 18 - \frac 19 + ... \\ & = \frac 2{\sqrt 3} \left(0 \cdot \frac 11 + \frac{\sqrt 3}2 \cdot \frac 12 - \frac{\sqrt 3}2 \cdot \frac 13 + 0 \cdot \frac 14 + \frac{\sqrt 3}2 \cdot \frac 15 - \frac{\sqrt 3}2 \cdot \frac 16 + 0 \cdot \frac 17 + \frac{\sqrt 3}2 \cdot \frac 18 - \frac{\sqrt 3}2 \cdot \frac 19 + ... \right) \\ & = \frac 2{\sqrt 3} \left( \sin 0 \cdot \frac 11 + \sin \frac{2\pi}3 \cdot \frac 12 - \sin \frac{4\pi}3 \cdot \frac 13 + \sin 2\pi \cdot \frac 14 + \sin \frac{8\pi}3 \cdot \frac 15 - \sin \frac{10\pi}3 \cdot \frac 16 + \sin 4\pi \cdot \frac 17 + \sin \frac{14\pi}3 \cdot \frac 18 - \sin \frac{16\pi}3 \cdot \frac 19 + ... \right) \\ & = \sum_{k=1}^\infty \frac{2\sin \left( (k-1)\frac {2 \pi}{3}\right)} {\sqrt{3} k} \end{aligned}

Can you explain how you got the second line?

Anthony Susevski - 5 years, 2 months ago

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Try writing out the first few values in the first and second lines. You'll see they are the same sum.

Ariel Gershon - 5 years, 2 months ago

Very clever solution! Good job!

Ariel Gershon - 5 years, 2 months ago

Sir,I saw that you have solved my problem named 'Area' (of calculus section) correctly.It would be very helpful if you could share your proof or atleast provide a brief outline of your solution as I am unable to solve the question.

Indraneel Mukhopadhyaya - 5 years, 2 months ago

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Sorry, I used numerical approach.

Chew-Seong Cheong - 5 years, 2 months ago

How did you got the second line ?

Aditya Sky - 4 years, 9 months ago

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I have added a note to explain it.

Chew-Seong Cheong - 4 years, 9 months ago

This is not a full solution . The calculation part is left for u guys. I have used taylor series and complex cube root of unity to solve the problem

Eto Boro korli keno ? Integration lagalei to hoto.

Prithwish Mukherjee - 2 years, 4 months ago

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