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 .
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First, let a = 2 0 1 5 , b = 2 0 1 6 , and c = 2 0 1 7
Then, break up the sum: i = 1 ∑ a ( j = 1 ∑ b ( k = 1 ∑ c ( j i ) + k = 1 ∑ c ( k j ) + k = 1 ∑ c ( i k ) ) ) =
i = 1 ∑ a ( j = 1 ∑ b ( k = 1 ∑ c ( j i ) ) ) + i = 1 ∑ a ( j = 1 ∑ b ( k = 1 ∑ c ( k j ) ) ) + i = 1 ∑ a ( j = 1 ∑ b ( k = 1 ∑ c ( i k ) ) )
Next, rearrange the order of the sums: i = 1 ∑ a ( j = 1 ∑ b ( k = 1 ∑ c ( j i ) ) ) + j = 1 ∑ b ( k = 1 ∑ c ( i = 1 ∑ a ( k j ) ) ) + i = 1 ∑ a ( k = 1 ∑ c ( j = 1 ∑ b ( i k ) ) )
This allows us to compute the innermost summations: i = 1 ∑ a ( j = 1 ∑ b ( c j i ) ) + j = 1 ∑ b ( k = 1 ∑ c ( a k j ) ) + i = 1 ∑ a ( k = 1 ∑ c ( b i k ) )
Rearranging again, we get: c j = 1 ∑ b ( i = 1 ∑ a ( j i ) ) + a k = 1 ∑ c ( j = 1 ∑ b ( k j ) + b i = 1 ∑ a ( k = 1 ∑ c ( i k ) ) ) =
c j = 1 ∑ b ⎝ ⎜ ⎛ j i = 1 ∑ a i ⎠ ⎟ ⎞ + a k = 1 ∑ c ⎝ ⎜ ⎛ k j = 1 ∑ b j ⎠ ⎟ ⎞ + b i = 1 ∑ a ⎝ ⎜ ⎛ i k = 1 ∑ c k ⎠ ⎟ ⎞
Using the summation formula n = 1 ∑ a a = 2 n ( n + 1 ) , we get c j = 1 ∑ b ( j 2 a ( a + 1 ) ) + a k = 1 ∑ c ( k 2 b ( b + 1 ) ) + b i = 1 ∑ a ( i 2 c ( c + 1 ) )
Factoring, we get c 2 a ( a + 1 ) j = 1 ∑ b ( j 1 ) + a 2 b ( b + 1 ) k = 1 ∑ c ( k 1 ) + b 2 c ( c + 1 ) i = 1 ∑ a ( i 1 ) =
2 1 a ( a + 1 ) c H b + 2 1 b ( b + 1 ) a H c + 2 1 c ( c + 1 ) b H a
Plugging it in, we see it is roughly equal to 1 0 0 6 6 2 4 9 0 3 6 4 , so our answer is 1 0 0 6 6