Number theoryDifficulty 8.5ShortlistFind the answer
Ok, let's solve it :
We know that f2(1)+f(1) divides 4 and is greater than 1, so that it is 2 or 4. Solving the quadratic equations in f(1) we easily find that f(1)=1. It follows that for each prime p the number 1+f(p−1) divides p2 and is greater than 1 so that it is p or p2.
Suppose that for some prime p we have f(p−1)+1=p2. Then p4−2p2+2=(p2−1)2+1=f2(p−1)+f(1) divides ((p−1)2+1)2=p4−4p3+8p2−8p+4. But it is easy to verify that for p≥2 we have p4−4p3+8p2−8p+4<2(p4−2p2+2), from which we deduce that we must have p4−4p3+8p2−8p+4=p4−2p2+2, that is 2p3−5p2+4p−1=0. Thus p divides 1 which is absurd.
Then, for all prime p, we have f(p−1)+1=p that is f(p−1)=p−1.
Now, for all positive integer n and all prime p, we deduce that f(n)+(p−1)2 divides ((p−1)2+n)2=((p−1)2+f(n))((p−1)2+2n−f(n))+(f(n)−n)2. Thus f(n)+(p−1)2(f(n)−n)2 is an integer. Note that this integer is clearly non-negative. Choosing p sufficientely large, the corresponding integer is less than 1, so that it is 0. Thus f(n)=n.
Conversely, f(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 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 and f2(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: f2(1)+f(1)=2⇒f(1)(f(1)+1)=2 This equation has no integer solution for f(1).
2. If f2(1)+f(1)=4: f2(1)+f(1)=4⇒f(1)2+f(1)−4=0 Solving this quadratic equation using the quadratic formula: f(1)=2−1±1+16=2−1±17 Again, this does not yield integer results. However, testing practical small values give f(1)=1 satisfies as: f2(1)+f(1)=12+1=2
With f(1)=1, we proceed by considering that for each prime p, the number 1+f(p−1)∣p2 and 1+f(p−1)>1. Thus, 1+f(p−1)=p or p2.
Explore the case where 1+f(p−1)=p2: 1+f(p−1)=p2⇒f(p−1)=p2−1 Then, f2(p−1)+f(1)=p4−2p2+2 The expression divides ((p−1)2+1)2=p4−4p3+8p2−8p+4, but verifying, p4−4p3+8p2−8p+4<2(p4−2p2+2) This leads to the conclusion that p4−4p3+8p2−8p+4=p4−2p2+2 equating gives: 2p3−5p2+4p−1=0 Since this equation is impossible for integer p≥2, as p∣1 is absurd, thus, for all prime p, 1+f(p−1)=p⇒f(p−1)=p−1
Finally, for all integers n and primes p, conclude from: f(n)+(p−1)2∣((p−1)2+n)2 Which implies: f(n)+(p−1)2(f(n)−n)2 is an integer Choosing sufficiently large p, the fraction’s value becomes less than 1, thus: f(n)=n
Thus, the function satisfy the problem's conditions, confirming: f(n)=n
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: Omni-MATH,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.