Let Q n + 1 ( x ) = Q ( Q n ( x ) ) , Q 1 ( x ) = Q ( x ) for all positive integers n .
Consider all polynomials Q ( x ) with integer coefficients such that Q 2 0 1 5 ( x ) − x has a positive integer root p . Find the minimum value of ( p + 1 ) 2 − Q 2 0 1 4 ( p ) .
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Q (x) might not be equal to x.
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But Q ( x ) = x is a requirement of the question, its just stating that if Q ( x ) = x , and that Q ( x ) is strictly increasing, the value of x satisfying Q ( x ) = x would be the same as Q 2 0 1 5 ( x ) = x
I'll change the variable to avoid confusion.
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How do you know Q 2 0 1 4 ( p ) = p ? The question did not state Q ( p ) = p .
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@Joel Tan – Root of Q 2 0 1 4 ( x ) − x = 0
Let the root be p
Q 2 0 1 4 ( p ) = p
I just assumed p = 1 and then proved it was possible.
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@Julian Poon – It is necessary to ensure that higher values of p do not give smaller answers.
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@Joel Tan – Its a quadratic whose minimum is when x = − 0 . 5 so it will be ever increasing if x is positive
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@Julian Poon – The problem is that Q(p) is not a constant.
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@Joel Tan – It is not constant, its strictly increasing.
I think I forgot to say that if Q ( x ) is strictly increasing or decreasing from x > p onward, and that one of the root for Q ( x ) − x = 0 is x = p , where p is a positive integer, one of the root for Q n ( x ) − x = 0 would be x = p , where n is a positive integer more that 1
Edit: wait thats different from what I typed but I intended to say this ^ and not the one in the solution
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@Julian Poon – I think there are too many assumptions made.
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@Joel Tan – Im rewriting my solution
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@Julian Poon – How do you get Q 2 0 1 4 ( p ) = p ? That is the first substitution you made.
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@Joel Tan – p is the root of the polynomial: Q 2 0 1 4 ( x ) − x = 0
So Q 2 0 1 4 ( p ) − p = 0 , Q 2 0 1 4 ( p ) = p
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@Julian Poon – It is Q 2 0 1 5 not Q 2 0 1 4 in the question
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@Joel Tan – f ( x ) − x = 0
f ( x ) = x
f ( f ( . . . . ( x ) . . . . ) ) = f ( x )
It doesn't matter.
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Since the value for this expression: ( p − 1 ) 2 − Q 2 0 1 4 ( p ) = ( p + 1 ) 2 − p is entirely dependent on p , we can assume the least value for p to get the answer.
Its a quadratic whose minimum is when x = − 0 . 5 so it will be ever increasing if x is positive. So we can make p = 1 and prove that it works.
I am going to sketch a prove that if Q ( x ) is strictly increasing or decreasing from x > p onward, and that one of the root for Q ( x ) − x = 0 is x = p , where p is a positive integer, one of the root for Q n ( x ) − x = 0 would be x = p , where n is a positive integer more that 1 .
When Q ( x ) is strictly increasing from x > p onward, when Q ( x ) > x || ( x > p ) , Q n + 1 ( x ) > Q n ( x )
When Q ( x ) < x || ( x < p ) , Q n + 1 ( x ) < Q n ( x )
So the equilibrium is reached when Q ( x ) = x , Q 2 0 1 5 ( x ) = Q ( x ) = x
Note that the degree for the polynomial Q ( x ) has to be more than 1 in order for this to work.
A similar thing can be done for the decreasing part.
A strictly increasing function Q ( x ) from x > 1 onward such that Q ( x ) − x = 0 has roots 1 can be Q ( x ) = x 2 , there are infinitely many solutions such as Q ( x ) = x 2 − 2 x + 2 . Anyways, trying out P n ( x ) − x = 0 , where P ( n ) = x 2 it indeed gives the root 1