Inspired by Dev Sharma

What is the remainder when k = 1 100 k 5 \displaystyle \sum_{k=1}^{100}k^5 is divided by 16.


Inspiration .


The answer is 4.

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1 solution

First we note that for any even k k we have that 16 k 5 . 16|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 ) . 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 , k, each of ( k 1 ) , ( k + 1 ) (k - 1), (k + 1) and ( k 2 + 1 ) (k^{2} + 1) is divisible by 2 , 2, and precisely one of ( k 1 ) , ( k + 1 ) (k - 1), (k + 1) is divisible by 4. 4. We can thus conclude that for odd k k

k 5 k 0 ( m o d 16 ) k 5 k ( m o d 16 ) . k^{5} - k \equiv 0 \pmod{16} \Longrightarrow k^{5} \equiv k \pmod{16}.

We can then say that the given sum S S is equivalent to the sum of all odd k k modulo 16 , 16, i.e.,

S = k = 1 100 k 5 n = 1 50 ( 2 n 1 ) 2 50 51 2 50 5 0 2 ( 3 16 + 2 ) 2 2 2 ( m o d 16 ) . S = \displaystyle\sum_{k=1}^{100} k^{5} \equiv \sum_{n=1}^{50} (2n - 1) \equiv 2*\dfrac{50*51}{2} - 50 \equiv 50^{2} \equiv (3*16 + 2)^{2} \equiv 2^{2} \pmod{16}.

We thus can conclude that the remainder when S S is divided by 16 16 is 4 . \boxed{4}.

Great again!

I basically recycled my solution here , adding that n λ ( 16 ) = n 4 1 ( m o d 16 ) n^{\lambda(16)}=n^4\equiv{1}\pmod{16} for odd n , n, so that n 5 n n^5\equiv{n} .

Otto Bretscher - 5 years, 9 months ago

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