A Game of Polynomials

Algebra Level 3

Written on a blackboard is the polynomial x 2 + x + 2014 x^2+x+2014 . Calvin and Peter take turns alternatively (starting with Calvin) in the following game. During his turn, Calvin should either increase or decrease the coefficient of x by 1 , and during his turn, Peter should either increase or decrease the constant coefficient by 1 . Calvin wins if at any point of time the polynomial on the blackboard at that instant has integer roots. Which of the following is true.

Calvin wins after an infinite number of steps Data insufficient Calvin can never win the game Calvin wins after a finite number of steps

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1 solution

Sergio Melo
Feb 22, 2020

We can modify the equation x 2 + x + 2014 { x }^{ 2 }+x+2014 with this: x 2 + C x + 2014 + P { x }^{ 2 }+Cx+2014+P , where C C is the total sum of Calvin and P P the sum of Peter. According to the equation, the roots are:

x = C ± C 2 8056 4 P 2 = C 2 ± C 2 8056 4 P 2 x=\frac { -C\pm \sqrt { { C }^{ 2 }-8056-4P } }{ 2 } =-\frac { C }{ 2 } \pm \frac { \sqrt { { C }^{ 2 }-8056-4P } }{ 2 }

Note that C C must be a multiple of 2 ( 2 n 2n ) and C 2 8056 4 P { C }^{ 2 }-8056-4P must be a perfect square ( ( a n ) 2 (a{ n })^{ 2 } ), where a a is a parameter (both a a and n n are integers too); with this, the roots are modified into this

x = 2 n 2 ± ( 2 n ) 2 8056 4 P 2 = n ± n 2 2014 P x=-\frac { 2n }{ 2 } \pm \frac { \sqrt { { (2n) }^{ 2 }-8056-4P } }{ 2 } =-n\pm \sqrt { { n }^{ 2 }-2014-P }

Given that C 2 8056 4 P { C }^{ 2 }-8056-4P or n 2 2014 P { n }^{ 2 }-2014-P must be a perfect square ( ( a n ) 2 (a{ n })^{ 2 } ), then

n 2 2014 P = ( a n ) 2 P = ( 1 a 2 ) n 2 2014 { n }^{ 2 }-2014-P={ (an) }^{ 2 }\longrightarrow P=(1-{ a }^{ 2 }){ n }^{ 2 }-2014 .

If we substitute this in the roots

n ± a n n ( 1 + a ) o r n ( a 1 ) -n\pm an\longrightarrow -n(1+a)\quad or\quad n(a-1) , given that C = 2 n C=2n , then the roots are C 2 -\frac{C}{2} ( 1 + a ) (1+a) or C 2 \frac{C}{2} ( a 1 ) (a-1)

With this (and the note below), Calvin could win in a finite number of steps.

Note: Look that P + 2014 = ( 1 a 2 ) n 2 P+2014=(1-{ a }^{ 2 }){ n }^{ 2 } , you can substitute this in the modified quadratic equation and you´ll get this

x 2 + C x + 2014 + P = x 2 + 2 n x + ( 1 a 2 ) n 2 { x }^{ 2 }+Cx+2014+P={ x }^{ 2 }+2nx+{ (1-a }^{ 2 }){ n }^{ 2 }

Solving the quadratic formula you can get the roots written above. You´ll get two integer solutions if a 0 a\neq 0 .

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