Function No. 2013

Algebra Level 4

Let f ( x ) = x 3 + a x 2 + b x + c f(x) = x^3 + ax^2 + bx + c and g ( x ) = x 3 + b x 2 + c x + a g(x) = x^3 + bx^2 + cx + a , where a , b , c a, b, c are integers with c 0 c\not = 0 . Suppose the following conditions hold :

  • f ( 1 ) = 0 f(1)= 0 .

  • The roots of g ( x ) g(x) are squares of roots of f ( x ) f(x) .

Find the value of a 2013 + b 2013 + c 2013 a^{2013}+b^{2013}+c^{2013} .


The answer is -1.

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2 solutions

Anish Harsha
Oct 19, 2015

Note that g ( 1 ) = f ( 1 ) = 0 g(1) = f(1) = 0 , so 1 is the root of both g ( x ) a n d f ( x ) g(x) and f(x) .
Let p and q are other two roots of f(x), so p 2 p^2 and q 2 q^2 are other two roots of g ( x ) g(x) . We then get c = p q -c = pq and a = p 2 q 2 -a = p^2 q^2 , so a = c 2 a = -c^2 .
Also ( a ) 2 = ( p + q + 1 ) 2 = p 2 + q 2 + 1 + 2 ( p q + q + p ) = b (-a)^2 = ( p + q+ 1)^2 = p^2 + q^2 + 1 + 2( pq + q + p ) = b .
Therefore b = c 4 b = c^4 . Since f ( 1 ) = 0 , c + 1 c 2 + c 4 = 0 f(1) = 0, c +1 - c^2 + c^4 = 0 . Factorizing, we get ( c + 1 ) ( c 3 c 2 + 1 ) = 0 ( c + 1) ( c^3 - c^2 + 1 ) = 0 .
Note that c 3 c 2 + 1 = 0 c^3 - c^2 + 1 = 0 has no integer root hence, c = 1 , b = 0 , a = 1 c = -1, b = 0 , a = -1 . So, a 2013 + b 2013 + c 2013 = 1 a^{2013} + b^{2013 }+ c^{2013} = -1 .


Something is not quite right here... c 3 c 2 + c c^3-c^2+c in the last line should be c 3 c 2 + 1 c^3-c^2+1 , I think.

And if c = a = 1 , b = 0 c = a = -1, b = 0 , the answer would be ( 1 ) + 0 + ( 1 ) = 2 (-1) + 0 + (-1) = -2 .

Arjen Vreugdenhil - 5 years, 7 months ago

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Sorry, I'll edit .

Anish Harsha - 5 years, 7 months ago

Your solution is very good, but you just need to fix a typo. It should say b = 1 b=1 . And for anyone else reading this, it might help to add a step: p 2 + q 2 + 1 + 2 ( p q + q + p ) = b + 2 b = b p^2+q^2+1+2(pq+q+p)=-b+2b=b .

James Wilson - 3 years, 6 months ago
James Wilson
Dec 3, 2017

We know that both f ( 1 ) = 0 f(1)=0 and g ( 1 ) = 0 g(1)=0 because 1 2 = 1 1^2=1 . Substituting 1 1 into either polynomial gives the same result, relating a a , b b , and c c : a + b + c + 1 = 0 a+b+c+1=0 . Then I use synthetic division to partially factor each polynomial as f ( x ) = ( x 1 ) ( x 2 + ( a + 1 ) x + a + b + 1 ) f(x)=(x-1)(x^2+(a+1)x+a+b+1) and g ( x ) = ( x 1 ) ( x 2 + ( b + 1 ) x + b + c + 1 ) g(x)=(x-1)(x^2+(b+1)x+b+c+1) . Using the relationship a + b + c + 1 = 0 a+b+c+1=0 , I can write g ( x ) = ( x 1 ) ( x 2 + ( b + 1 ) x a ) g(x)=(x-1)(x^2+(b+1)x-a) , eliminating the variable c c . Let α \alpha and β \beta be the other roots of f ( x ) f(x) --other than the known root x = 1 x=1 . Then through applying Vieta's formulas to both f ( x ) f(x) and g ( x ) g(x) , I get the four equations: α + β = a 1 , α β = a + b + 1 , α 2 + β 2 = b 1 , α 2 β 2 = a \alpha+\beta=-a-1,\alpha\beta=a+b+1,\alpha^2+\beta^2=-b-1,\alpha^2\beta^2=-a . I first square the second equation, so that I can equate it with the fourth: α 2 β 2 = a = a 2 + b 2 + 2 a b + 2 a + 2 b + 1 \alpha^2\beta^2=-a=a^2+b^2+2ab+2a+2b+1 .Then I square the first, multiply the second by 2 2 , subtract the resulting equations, and equate it with the third equation to get: α 2 + β 2 = a 2 2 b 1 = b 1 \alpha^2+\beta^2=a^2-2b-1=-b-1 , which implies a 2 = b a^2=b . Then after substituting a 2 = b a^2=b into a 2 + b 2 + 2 a b + 3 a + 2 b + 1 = 0 a^2+b^2+2ab+3a+2b+1=0 , and simplifying, I get a 4 + 2 a 3 + 3 a 2 + 3 a + 1 = 0 a^4+2a^3+3a^2+3a+1=0 . There is only one integer root of this polynomial (cf. https://brilliant.org/wiki/rational-root-theorem/), which leads to an answer of a = 1 , b = 1 , c = 1 a=-1, b=1, c=-1 . Hence, a 2013 + b 2013 + c 2013 = ( 1 ) 2013 + ( 1 ) 2013 + ( 1 ) 2013 = 1 a^{2013}+b^{2013}+c^{2013}=(-1)^{2013}+(1)^{2013}+(-1)^{2013}=-1 .

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