Bet you can't bash this out!

Algebra Level 5

Given that z , n , p z, n, p stand for the number of non-real, negative and non-negative roots of the equation 2014 x 2014 + 3021 x 1006 4028 x = 0 2014x^{2014}+3021x^{1006}-4028x=0 , find the value of ( z × 3 p ) 2014 ( m o d 10000 ) (z \times 3p)^{2014} \pmod {10000}


The answer is 2944.

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1 solution

Krishna Ar
Sep 14, 2014

The problem is a simple application of Descartes' rule of signs

Lets take this poly to be f ( x ) = 2014 x 2014 + 3021 x 1006 4028 x f(x)=2014x^{2014}+3021x^{1006}-4028x .

We find that it can be factored to 1007 x ( 2 x 2013 + 3 x 1005 4 ) . 1007x(2x^{2013}+3x^{1005}-4). Thus one root is x = 0 x=0 .

Let g ( x ) = 2 x 2013 + 3 x 1005 4 g(x)=2x^{2013}+3x^{1005}-4

\bullet g ( x ) g(x) has 1 1 sign change, in the coefficient changing from 3 3 to 4 -4 . Thus g ( x ) g(x) has 1 positive root and including 0, there are 2 p = 2 \implies p=2

\bullet g ( x ) = 2 x 2013 3 x 1005 4 g(-x)=-2x^{2013}-3x^{1005}-4 , this has 0 0 sign changes, to give us 0 0 negative roots.

\bullet Thus it has 2012 2012 complex roots. (2014 total roots-2 positive real roots) z = 2012 \implies z=2012


Now all we gotta do is to simply simplify the above expression 1207 2 2014 ( m o d 10000 ) 12072^{2014} \pmod {10000} which is a piece of cake using Euler's totient theorem.

Answer is 2944 \boxed{2944}

If you liked my solution-Kindly upvote it. :)

This problem makes use of Signs logic/rules which are easy to catch from the above explanation (hopefully ) :D

Krishna Ar - 6 years, 9 months ago

Note that the real numbers are a subset of the complex numbers. Thus, there are 2014 complex roots.

I've updated the phrasing of the problem to reflect what you intended.

Calvin Lin Staff - 6 years, 8 months ago

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Thanks for that! The wording nearly killed me! :D

Krishna Ar - 6 years, 8 months ago

Great application of Decartes' Rule of Signs!!

Nathan Ramesh - 6 years, 7 months ago

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Thank you very much :) ...Did you do it the same way too?

Krishna Ar - 6 years, 7 months ago

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I used a calculator for about 15 min to calculate the remainder, hence I bashed this out with a calculator.

Ronak Agarwal - 6 years, 6 months ago

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@Ronak Agarwal LOL Hope you didnt use it for the 1st part :P

Krishna Ar - 6 years, 6 months ago

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