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Number theory Difficulty 8.5 Shortlist Find the answer

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We know that f2(1)+f(1)f^2(1)+f(1) divides 44 and is greater than 11, so that it is 22 or 44. Solving the quadratic equations in f(1)f(1) we easily find that f(1)=1.f(1)=1.
It follows that for each prime pp the number 1+f(p1)1+f(p-1) divides p2p^2 and is greater than 11 so that it is pp or p2p^2.

Suppose that for some prime pp we have f(p1)+1=p2.f(p-1)+1 = p^2.
Then p42p2+2=(p21)2+1=f2(p1)+f(1)p^4-2p^2+2 = (p^2-1)^2 + 1 = f^2(p-1)+f(1) divides ((p1)2+1)2=p44p3+8p28p+4((p-1)^2+1)^2 = p^4-4p^3 + 8p^2 - 8p +4.
But it is easy to verify that for p2p \geq 2 we have p44p3+8p28p+4<2(p42p2+2)p^4-4p^3 + 8p^2 - 8p +4 <2(p^4-2p^2+2), from which we deduce that we must have p44p3+8p28p+4=p42p2+2p^4-4p^3 + 8p^2 - 8p +4 = p^4 - 2p^2 + 2, that is 2p35p2+4p1=02p^3-5p^2+4p-1=0. Thus pp divides 11 which is absurd.

Then, for all prime pp, we have f(p1)+1=pf(p-1)+1=p that is f(p1)=p1.f(p-1)=p-1.

Now, for all positive integer nn and all prime pp, we deduce that f(n)+(p1)2f(n)+(p-1)^2 divides ((p1)2+n)2=((p1)2+f(n))((p1)2+2nf(n))+(f(n)n)2((p-1)^2+n)^2 = ((p-1)^2+f(n))((p-1)^2 + 2n - f(n)) + (f(n) - n)^2.
Thus (f(n)n)2f(n)+(p1)2\frac {(f(n)-n)^2} {f(n) + (p-1)^2} is an integer.
Note that this integer is clearly non-negative. Choosing pp sufficientely large, the corresponding integer is less than 11, so that it is 00. Thus f(n)=nf(n) = n.

Conversely, f(n)=nf(n)=n is clearly a solution of the problem.

Pierre.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Let us find a function f f such that the conditions given in the problem statement are satisfied, starting from given hints and systematically addressing each part of the problem.

First, we analyze the condition f2(1)+f(1)4 f^2(1) + f(1) \mid 4 and f2(1)+f(1)>1 f^2(1) + f(1) > 1 . Since divisors of 4 greater than 1 are 2 and 4, we can set:

1. If f2(1)+f(1)=2 f^2(1) + f(1) = 2 :
f2(1)+f(1)=2f(1)(f(1)+1)=2 f^2(1) + f(1) = 2 \quad \Rightarrow \quad f(1)(f(1) + 1) = 2
This equation has no integer solution for f(1) f(1) .

2. If f2(1)+f(1)=4 f^2(1) + f(1) = 4 :
f2(1)+f(1)=4f(1)2+f(1)4=0 f^2(1) + f(1) = 4 \quad \Rightarrow \quad f(1)^2 + f(1) - 4 = 0
Solving this quadratic equation using the quadratic formula:
f(1)=1±1+162=1±172 f(1) = \frac{-1 \pm \sqrt{1 + 16}}{2} = \frac{-1 \pm \sqrt{17}}{2}
Again, this does not yield integer results. However, testing practical small values give f(1)=1 f(1) = 1 satisfies as:
f2(1)+f(1)=12+1=2 f^2(1) + f(1) = 1^2 + 1 = 2

With f(1)=1 f(1) = 1 , we proceed by considering that for each prime p p , the number 1+f(p1)p2 1 + f(p-1) \mid p^2 and 1+f(p1)>1 1 + f(p-1) > 1 . Thus, 1+f(p1)=p 1 + f(p-1) = p or p2 p^2 .

Explore the case where 1+f(p1)=p2 1 + f(p-1) = p^2 :
1+f(p1)=p2f(p1)=p21 1 + f(p-1) = p^2 \quad \Rightarrow \quad f(p-1) = p^2 - 1
Then,
f2(p1)+f(1)=p42p2+2 f^2(p-1) + f(1) = p^4 - 2p^2 + 2
The expression divides ((p1)2+1)2=p44p3+8p28p+4 ((p-1)^2 + 1)^2 = p^4 - 4p^3 + 8p^2 - 8p + 4 , but verifying,
p44p3+8p28p+4<2(p42p2+2) p^4 - 4p^3 + 8p^2 - 8p + 4 < 2(p^4 - 2p^2 + 2)
This leads to the conclusion that p44p3+8p28p+4=p42p2+2 p^4 - 4p^3 + 8p^2 - 8p + 4 = p^4 - 2p^2 + 2 equating gives:
2p35p2+4p1=0 2p^3 - 5p^2 + 4p - 1 = 0
Since this equation is impossible for integer p2 p \geq 2 , as p1 p \mid 1 is absurd, thus, for all prime p p ,
1+f(p1)=pf(p1)=p1 1 + f(p-1) = p \quad \Rightarrow \quad f(p-1) = p - 1

Finally, for all integers n n and primes p p , conclude from:
f(n)+(p1)2((p1)2+n)2 f(n) + (p-1)^2 \mid ((p-1)^2 + n)^2
Which implies:
(f(n)n)2f(n)+(p1)2 is an integer \frac{(f(n) - n)^2}{f(n) + (p-1)^2} \text{ is an integer}
Choosing sufficiently large p p , the fraction’s value becomes less than 1, thus:
f(n)=n f(n) = n

Thus, the function satisfy the problem's conditions, confirming:
f(n)=n \boxed{f(n) = n}

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