Olympiad Maths Prep

Track / Stage 7 / 61 of 300 #1461 of 2000

Problem 1461

National olympiad second round; IMO P1/P4
Number theory Difficulty 7.1 Prove it

Let pp be a prime number of the form 9k+19k + 1. Show that there exists an integer n such that pn33n+1p | n^3 - 3n + 1.

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

1. Given: p p is a prime number of the form 9k+1 9k + 1 . This implies that p1(mod9) p \equiv 1 \pmod{9} .

2. Objective: Show that there exists an integer n n such that pn33n+1 p \mid n^3 - 3n + 1 .

3. Step 1: Since p1(mod9) p \equiv 1 \pmod{9} , we know that 9(p1) 9 \mid (p - 1) . This implies that p1=9m p - 1 = 9m for some integer m m .

4. Step 2: Consider the finite field Fp2 \mathbb{F}_{p^2} . Since p1(mod9) p \equiv 1 \pmod{9} , the multiplicative group Fp2 \mathbb{F}_{p^2}^* has order p21 p^2 - 1 .

5. Step 3: The order of Fp2 \mathbb{F}_{p^2}^* is p21 p^2 - 1 . Since p1(mod9) p \equiv 1 \pmod{9} , we have p210(mod9) p^2 - 1 \equiv 0 \pmod{9} . Therefore, 9(p21) 9 \mid (p^2 - 1) .

6. Step 4: There exists an element t t in Fp2 \mathbb{F}_{p^2} with order 9. This is because the order of the multiplicative group Fp2 \mathbb{F}_{p^2}^* is divisible by 9.

7. Step 5: Consider the element t+1t t + \frac{1}{t} in Fp \mathbb{F}_p . We need to show that t+1t t + \frac{1}{t} satisfies the equation n33n+10(modp) n^3 - 3n + 1 \equiv 0 \pmod{p} .

8. Step 6: Compute (t+1t)33(t+1t)+1 \left(t + \frac{1}{t}\right)^3 - 3\left(t + \frac{1}{t}\right) + 1 :
(t+1t)3=t3+3t+3t+1t3 \left(t + \frac{1}{t}\right)^3 = t^3 + 3t + \frac{3}{t} + \frac{1}{t^3}
(t+1t)33(t+1t)+1=t3+1t3+3(t+1t)3(t+1t)+1 \left(t + \frac{1}{t}\right)^3 - 3\left(t + \frac{1}{t}\right) + 1 = t^3 + \frac{1}{t^3} + 3\left(t + \frac{1}{t}\right) - 3\left(t + \frac{1}{t}\right) + 1
=t3+1t3+1 = t^3 + \frac{1}{t^3} + 1

9. Step 7: Since t t has order 9, t9=1 t^9 = 1 . Therefore, t6=1t3 t^6 = \frac{1}{t^3} . Substitute t6=1t3 t^6 = \frac{1}{t^3} into the equation:
t3+1t3+1=t3+t6+1=t3+1t3+1=0 t^3 + \frac{1}{t^3} + 1 = t^3 + t^6 + 1 = t^3 + \frac{1}{t^3} + 1 = 0

10. Step 8: Thus, (t+1t)33(t+1t)+1=0 \left(t + \frac{1}{t}\right)^3 - 3\left(t + \frac{1}{t}\right) + 1 = 0 . Therefore, we can take nt+1t(modp) n \equiv t + \frac{1}{t} \pmod{p} .

Conclusion:
nt+1t(modp) \boxed{n \equiv t + \frac{1}{t} \pmod{p}}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.