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First we note that for any even k we have that 1 6 ∣ k 5 .
Next, we observe that k 5 − k = k ( k 4 − 1 ) = k ( k 2 − 1 ) ( k 2 + 1 ) = k ( k − 1 ) ( k + 1 ) ( k 2 + 1 ) . For odd k , each of ( k − 1 ) , ( k + 1 ) and ( k 2 + 1 ) is divisible by 2 , and precisely one of ( k − 1 ) , ( k + 1 ) is divisible by 4 . We can thus conclude that for odd k
k 5 − k ≡ 0 ( m o d 1 6 ) ⟹ k 5 ≡ k ( m o d 1 6 ) .
We can then say that the given sum S is equivalent to the sum of all odd k modulo 1 6 , i.e.,
S = k = 1 ∑ 1 0 0 k 5 ≡ n = 1 ∑ 5 0 ( 2 n − 1 ) ≡ 2 ∗ 2 5 0 ∗ 5 1 − 5 0 ≡ 5 0 2 ≡ ( 3 ∗ 1 6 + 2 ) 2 ≡ 2 2 ( m o d 1 6 ) .
We thus can conclude that the remainder when S is divided by 1 6 is 4 .