Is it even possible?

Algebra Level 5

A 2015th degree polynomial f ( x ) f(x) satisfies the equation f ( k ) = 2 k f(k) = 2^k for k = 0 , 1 , 2 , . . . , 2015 k = 0, 1, 2,..., 2015 . Find the remainder when f ( 2016 ) f(2016) is divided by 7.


The answer is 0.

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1 solution

Kushal Dey
Jan 3, 2021

Let g(x)=x(x-1)(x-2)....(x-n). Thus f(x) can be assumed as k = 0 n \sum_{k=0}^n a k a_{k} 2 k 2^{k} g(x)/(x-k). Now we can choose the values of f(k) with the help of companion function g(x). Now obviously we will choose a k a_{k} in a way that f(k)= 2 k 2^{k} . It's not difficult to see that a k a_{k} = ( 1 ) k + 1 (-1)^{k+1} 2015Ck/2015! (where nCr is the coefficient of x r x^{r} in expansion of ( 1 + x ) n (1+x)^{n} ). Now put x=n+1 in f(x), it becomes k = 0 n \sum_{k=0}^n ( 1 ) k + 1 (-1)^{k+1} (n+1)Ck 2 k 2^{k} which simplifies to 2 n + 1 2^{n+1} -1. Put n=2015, and you have f(2016)= 2 2016 2^{2016} -1= 8 672 8^{672} -1=(8-1)*( 8 671 8^{671} + 8 670 8^{670} +...+8²+8+1), clearly a multiple of 7.

This is my first explanation using latex :)

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