Polynomials and more polynomials?

Algebra Level 5

Let Q n + 1 ( x ) = Q ( Q n ( x ) ) , Q 1 ( x ) = Q ( x ) Q_{n+1}(x)=Q (Q _{n}(x)), Q_{1}(x)=Q (x) for all positive integers n n .

Consider all polynomials Q ( x ) Q (x) with integer coefficients such that Q 2015 ( x ) x Q_{2015} (x)-x has a positive integer root p p . Find the minimum value of ( p + 1 ) 2 Q 2014 ( p ) (p+1)^{2}-Q_{2014}(p) .


The answer is 3.

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1 solution

Julian Poon
Feb 25, 2015

Since the value for this expression: ( p 1 ) 2 Q 2014 ( p ) = ( p + 1 ) 2 p (p-1)^{2} - Q_{2014}(p)=\left(p+1\right)^2-p is entirely dependent on p p , we can assume the least value for p p to get the answer.

Its a quadratic whose minimum is when x = 0.5 x=-0.5 so it will be ever increasing if x x is positive. So we can make p = 1 p=1 and prove that it works.

I am going to sketch a prove that if Q ( x ) Q(x) is strictly increasing or decreasing from x > p x>p onward, and that one of the root for Q ( x ) x = 0 Q(x)-x=0 is x = p x=p , where p p is a positive integer, one of the root for Q n ( x ) x = 0 Q_{n}(x) - x = 0 would be x = p x=p , where n n is a positive integer more that 1 1 .

When Q ( x ) Q(x) is strictly increasing from x > p x>p onward, when Q ( x ) > x Q(x)>x || ( x > p ) (x>p) , Q n + 1 ( x ) > Q n ( x ) Q_{n+1}(x)>Q_{n}(x)

When Q ( x ) < x Q(x)<x || ( x < p ) (x<p) , Q n + 1 ( x ) < Q n ( x ) Q_{n+1}(x)<Q_{n}(x)

So the equilibrium is reached when Q ( x ) = x Q(x)=x , Q 2015 ( x ) = Q ( x ) = x Q_{2015}(x)=Q(x)=x

Note that the degree for the polynomial Q ( x ) Q(x) has to be more than 1 1 in order for this to work.

A similar thing can be done for the decreasing part.

A strictly increasing function Q ( x ) Q(x) from x > 1 x>1 onward such that Q ( x ) x = 0 Q(x)-x=0 has roots 1 1 can be Q ( x ) = x 2 Q(x)=x^{2} , there are infinitely many solutions such as Q ( x ) = x 2 2 x + 2 Q(x)=x^2-2x+2 . Anyways, trying out P n ( x ) x = 0 P_{n}(x)-x=0 , where P ( n ) = x 2 P(n)=x^{2} it indeed gives the root 1 1

Q (x) might not be equal to x.

Joel Tan - 6 years, 3 months ago

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But Q ( x ) = x Q(x)=x is a requirement of the question, its just stating that if Q ( x ) = x Q(x)=x , and that Q ( x ) Q(x) is strictly increasing, the value of x x satisfying Q ( x ) = x Q(x)=x would be the same as Q 2015 ( x ) = x Q_{2015}(x)=x

I'll change the variable to avoid confusion.

Julian Poon - 6 years, 3 months ago

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How do you know Q 2014 ( p ) = p Q_{2014}(p)=p ? The question did not state Q ( p ) = p Q (p) =p .

Joel Tan - 6 years, 3 months ago

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@Joel Tan Root of Q 2014 ( x ) x = 0 Q_{2014}(x)-x=0

Let the root be p p

Q 2014 ( p ) = p Q_{2014}(p)=p

I just assumed p = 1 p=1 and then proved it was possible.

Julian Poon - 6 years, 3 months ago

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@Julian Poon It is necessary to ensure that higher values of p do not give smaller answers.

Joel Tan - 6 years, 3 months ago

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@Joel Tan Its a quadratic whose minimum is when x = 0.5 x=-0.5 so it will be ever increasing if x x is positive

Julian Poon - 6 years, 3 months ago

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@Julian Poon The problem is that Q(p) is not a constant.

Joel Tan - 6 years, 3 months ago

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@Joel Tan It is not constant, its strictly increasing.

I think I forgot to say that if Q ( x ) Q(x) is strictly increasing or decreasing from x > p x>p onward, and that one of the root for Q ( x ) x = 0 Q(x)-x=0 is x = p x=p , where p p is a positive integer, one of the root for Q n ( x ) x = 0 Q_{n}(x) - x = 0 would be x = p x=p , where n n is a positive integer more that 1 1

Edit: wait thats different from what I typed but I intended to say this ^ and not the one in the solution

Julian Poon - 6 years, 3 months ago

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@Julian Poon I think there are too many assumptions made.

Joel Tan - 6 years, 3 months ago

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@Joel Tan Im rewriting my solution

Julian Poon - 6 years, 3 months ago

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@Julian Poon How do you get Q 2014 ( p ) = p Q_{2014}(p)=p ? That is the first substitution you made.

Joel Tan - 6 years, 3 months ago

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@Joel Tan p p is the root of the polynomial: Q 2014 ( x ) x = 0 Q_{2014}(x)-x=0

So Q 2014 ( p ) p = 0 Q_{2014}(p)-p=0 , Q 2014 ( p ) = p Q_{2014}(p)=p

Julian Poon - 6 years, 3 months ago

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@Julian Poon It is Q 2015 Q_{2015} not Q 2014 Q_{2014} in the question

Joel Tan - 6 years, 3 months ago

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@Joel Tan f ( x ) x = 0 f(x)-x=0

f ( x ) = x f(x)=x

f ( f ( . . . . ( x ) . . . . ) ) = f ( x ) f(f(....(x)....))=f(x)

It doesn't matter.

Julian Poon - 6 years, 3 months ago

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