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Algebra Level 5

i = 1 2015 j = 1 2016 k = 1 2017 ( i j + j k + k i ) \large \sum _{i=1}^{2015}\sum _{j=1}^{2016}\sum _{k=1}^{2017}\left(\dfrac{i}{j}+\dfrac{j}{k}+\dfrac{k}{i}\right)

Find the first 5 digits of the sum above.

Note : You may use a 4-operation calculator and look up the values of the harmonic numbers .


The answer is 10066.

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1 solution

Matthew Riedman
May 5, 2016

First, let a = 2015 , b = 2016 a=2015, b=2016 , and c = 2017 c=2017

Then, break up the sum: i = 1 a ( j = 1 b ( k = 1 c ( i j ) + k = 1 c ( j k ) + k = 1 c ( k i ) ) ) = \large \sum\limits _{i=1}^a\left(\sum \limits_{j=1}^b\left(\sum \limits_{k=1}^c\left(\frac{i}{j}\right)+\sum \limits_{k=1}^c\left(\frac{j}{k}\right)+\sum \limits_{k=1}^c\left(\frac{k}{i}\right)\right)\right)=

i = 1 a ( j = 1 b ( k = 1 c ( i j ) ) ) + i = 1 a ( j = 1 b ( k = 1 c ( j k ) ) ) + i = 1 a ( j = 1 b ( k = 1 c ( k i ) ) ) \large \sum \limits_{i=1}^a\left(\sum \limits_{j=1}^b\left(\sum \limits_{k=1}^c\left(\frac{i}{j}\right)\right)\right)+\sum \limits_{i=1}^a\left(\sum \limits_{j=1}^b\left(\sum \limits_{k=1}^c\left(\frac{j}{k}\right)\right)\right)+\sum \limits_{i=1}^a\left(\sum \limits_{j=1}^b\left(\sum \limits_{k=1}^c\left(\frac{k}{i}\right)\right)\right)

Next, rearrange the order of the sums: i = 1 a ( j = 1 b ( k = 1 c ( i j ) ) ) + j = 1 b ( k = 1 c ( i = 1 a ( j k ) ) ) + i = 1 a ( k = 1 c ( j = 1 b ( k i ) ) ) \large \sum \limits_{i=1}^a\left(\sum\limits _{j=1}^b\left(\sum \limits_{k=1}^c\left(\frac{i}{j}\right)\right)\right)+\sum \limits_{j=1}^b\left(\sum \limits_{k=1}^c\left(\sum \limits_{i=1}^a\left(\frac{j}{k}\right)\right)\right)+\sum \limits_{i=1}^a\left(\sum\limits _{k=1}^c\left(\sum \limits_{j=1}^b\left(\frac{k}{i}\right)\right)\right)

This allows us to compute the innermost summations: i = 1 a ( j = 1 b ( c i j ) ) + j = 1 b ( k = 1 c ( a j k ) ) + i = 1 a ( k = 1 c ( b k i ) ) \large \sum \limits_{i=1}^a\left(\sum \limits_{j=1}^b\left(c\frac{i}{j}\right)\right)+\sum \limits_{j=1}^b\left(\sum \limits_{k=1}^c\left(a\frac{j}{k}\right)\right)+\sum \limits_{i=1}^a\left(\sum \limits_{k=1}^c\left(b\frac{k}{i}\right)\right)

Rearranging again, we get: c j = 1 b ( i = 1 a ( i j ) ) + a k = 1 c ( j = 1 b ( j k ) + b i = 1 a ( k = 1 c ( k i ) ) ) = \large c\sum \limits_{j=1}^b\left(\sum \limits_{i=1}^a\left(\frac{i}{j}\right)\right)+a\sum \limits_{k=1}^c\left(\sum \limits_{j=1}^b\left(\frac{j}{k}\right)+b\sum \limits_{i=1}^a\left(\sum \limits_{k=1}^c\left(\frac{k}{i}\right)\right)\right)=

c j = 1 b ( i = 1 a i j ) + a k = 1 c ( j = 1 b j k ) + b i = 1 a ( k = 1 c k i ) \large c\sum \limits_{j=1}^b\left(\frac{\sum \limits_{i=1}^ai}{j}\right)+a\sum \limits_{k=1}^c\left(\frac{\sum \limits_{j=1}^bj}{k}\right)+b\sum \limits_{i=1}^a\left(\frac{\sum \limits_{k=1}^ck}{i}\right)

Using the summation formula n = 1 a a = n ( n + 1 ) 2 \large \sum\limits _{n=1}^aa=\frac{n\left(n+1\right)}{2} , we get c j = 1 b ( a ( a + 1 ) 2 j ) + a k = 1 c ( b ( b + 1 ) 2 k ) + b i = 1 a ( c ( c + 1 ) 2 i ) c\sum \limits_{j=1}^b\left(\frac{\frac{a\left(a+1\right)}{2}}{j}\right)+a\sum \limits_{k=1}^c\left(\frac{\frac{b\left(b+1\right)}{2}}{k}\right)+b\sum \limits_{i=1}^a\left(\frac{\frac{c\left(c+1\right)}{2}}{i}\right)

Factoring, we get c a ( a + 1 ) 2 j = 1 b ( 1 j ) + a b ( b + 1 ) 2 k = 1 c ( 1 k ) + b c ( c + 1 ) 2 i = 1 a ( 1 i ) = \large c\frac{a\left(a+1\right)}{2}\sum \limits_{j=1}^b\left(\frac{1}{j}\right)+a\frac{b\left(b+1\right)}{2}\sum \limits_{k=1}^c\left(\frac{1}{k}\right)+b\frac{c\left(c+1\right)}{2}\sum \limits_{i=1}^a\left(\frac{1}{i}\right)=

1 2 a ( a + 1 ) c H b + 1 2 b ( b + 1 ) a H c + 1 2 c ( c + 1 ) b H a \frac{1}{2}a\left(a+1\right)cH_b+\frac{1}{2}b\left(b+1\right)aH_c+\frac{1}{2}c\left(c+1\right)bH_a

Plugging it in, we see it is roughly equal to 100662490364 100662490364 , so our answer is 10066 \boxed{10066}

I did exactly the same. Is there another way of solving it? I would really like to see that. That was a very nice problem.

Raz Lerman - 5 years, 1 month ago

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