New Year's Algebra

Algebra Level 5

2017 f ( x ) + 2016 f ( x ) = ( 2016 + 2017 ) k = 0 2017 x k 2017f(x)+2016f(-x)=\left(2016+2017 \right) \sum_{k=0}^{2017}x^{k}

f ( x ) f(x) is a polynomial function of degree 2017 that satisfies the above functional equation for all x C x \in \mathbb{C} .

Let R R be the sum of the distinct real roots of f ( x ) f(x) and C C be the sum of the distinct non-real roots of f ( x ) f(x) .

If the value of C R C-R can be expressed in the form p q \dfrac{p}{q} , for coprime positive integers p , q p, q , find p + q p+q .

Clarification: For any complex number z = a + b i z=a+bi where i = 1 i=\sqrt{-1} and a , b a,b are real, z z is considered real for b = 0 b=0 and non-real for b 0 b \ne 0 .


The answer is 4034.

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1 solution

Brandon Monsen
Dec 31, 2016

Let f ( x ) = g ( x ) + h ( x ) f(x)=g(x)+h(x) , where g ( x ) g(x) contains only the odd powers of x x in f ( x ) f(x) and h ( x ) h(x) contains only the even powers of x x in f ( x ) f(x) . f ( x ) = g ( x ) + h ( x ) \Rightarrow f(-x)=-g(x)+h(x) .

Plugging this into the functional equation:

g ( x ) + 4033 h ( x ) = 4033 ( 1 + x + x 2 + x 3 + . . . + x 2016 + x 2017 ) g(x)+4033h(x)=4033\left( 1+x+x^{2}+x^{3}+...+x^{2016}+x^{2017} \right)

We can now assign the even/odd powers of x x to g ( x ) g(x) and h ( x ) h(x) based on their definitions:

g ( x ) = 4033 ( x + x 3 + x 5 + . . . + x 2015 + x 2017 ) h ( x ) = 1 + x 2 + x 4 + x 6 + . . . + x 2014 + x 2016 \begin{array}{c}&g(x)=4033\left( x+x^{3}+x^{5}+...+x^{2015}+x^{2017} \right) \\ h(x)=1+x^{2}+x^{4}+x^{6}+...+x^{2014}+x^{2016} \end{array}

Solving for f ( x ) : f(x):

f ( x ) = 4033 ( x + x 3 + x 5 + . . . + x 2015 + x 2017 ) + 1 + x 2 + x 4 + x 6 + . . . + x 2014 + x 2016 f ( x ) = ( 4033 x + 1 ) ( 1 + x 2 + x 4 + x 6 + . . . + x 2014 + x 2016 ) \begin{array}{c}& f(x)=4033 \left( x+x^{3}+x^{5}+...+x^{2015}+x^{2017} \right) + 1+x^{2}+x^{4}+x^{6}+...+x^{2014}+x^{2016} \\ f(x)= \left( 4033x+1 \right) \left(1+x^{2}+x^{4}+x^{6}+...+x^{2014}+x^{2016} \right) \end{array}

We now have f ( x ) f(x) in the form f ( x ) = i ( x ) j ( x ) f(x)=i(x)j(x) where i ( x ) = 4033 x + 1 i(x)=4033x+1 and j ( x ) = 1 + x 2 + x 4 + x 6 + . . . + x 2014 + x 2016 j(x)=1+x^{2}+x^{4}+x^{6}+...+x^{2014}+x^{2016}

  • i ( x ) x = 1 4033 i(x) \rightarrow x=-\frac{1}{4033} is a root.

  • j ( x ) j ( x ) ( 1 x 2 ) = 1 x 2018 j(x) \rightarrow j(x)\left( 1-x^{2}\right)=1-x^{2018} so the roots of j ( x ) j(x) are the 201 8 t h 2018^{th} roots of unity excluding 1 1 and 1 -1 , all of which are non-real, distinct, and sum to 0 0 by Vieta's Formula .

C = 0 , R = 1 4033 C R = 1 4033 p + q = 4034 \Rightarrow C=0, R=-\frac{1}{4033} \Rightarrow C-R=\frac{1}{4033} \Rightarrow p+q=\boxed{4034}

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