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Number theory Difficulty 5.6 AIME, harder Prove it Silk Road Mathematics Competition

Determine all prime numbers pp for which there are integers mm and nn such that p=m2+n2p = m^2 + n^2 and p∣m3+n3−4p \mid m^3 + n^3 - 4.

Solution

Considering all integers ∣m∣,∣n∣≤3|m|, |n| \le 3 we can get the solutions 2=12+122 = 1^2 + 1^2, 5=12+225 = 1^2 + 2^2 and 13=(−3)2+(−2)213 = (-3)^2 + (-2)^2. Now, let's prove that there is no other prime numbers satisfying the statement. We have
p=m2+n2⇒p=(m+n)2−2mn⇒mn=(m+n)2−p2, p = m^2 + n^2 \Rightarrow p = (m + n)^2 - 2mn \Rightarrow mn = \frac{(m + n)^2 - p}{2},
m3+n3−4=(m+n)3−3mn(m+n)−4=−(m+n)3+3p(m+n)−82, m^3 + n^3 - 4 = (m + n)^3 - 3mn(m + n) - 4 = \frac{-(m + n)^3 + 3p(m + n) - 8}{2},
p∣m3+n3−4⇒p∣(m+n)3+8. p \mid m^3 + n^3 - 4 \Rightarrow p \mid (m + n)^3 + 8.
Hence, p∣m+n+2p \mid m+n+2 or p∣(m+n)2−2(m+n)+4p \mid (m+n)^2 - 2(m+n) + 4, and from last we conclude that p∣mn−m−n+2p \mid mn - m - n + 2, if we set p>2p > 2.
In a first case we have:
p∣m+n+2⇒m2+n2≤∣m+n+2∣, p \mid m + n + 2 \Rightarrow m^2 + n^2 \le |m + n + 2|,
i.e.
m2+n2≤m+n+2⇒(2m−1)2+(2n−1)2≤10⇒−1≤m,n≤2. m^2 + n^2 \le m + n + 2 \Rightarrow (2m - 1)^2 + (2n - 1)^2 \le 10 \Rightarrow -1 \le m, n \le 2.
m2+n2≤−(m+n+2)⇒(2m+1)2+(2n+1)2≤−6, m^2 + n^2 \le -(m+n+2) \Rightarrow (2m+1)^2 + (2n+1)^2 \le -6,
which is a contradiction.
In a second case:
p∣mn−m−n+2⇒m2+n2≤∣mn−m−n+2∣, p \mid mn - m - n + 2 \Rightarrow m^2 + n^2 \le |mn - m - n + 2|,
i.e.
m2+n2≤mn−m−n+2⇒(2m−n+1)2+3(n+1)2≤12⇒−3≤m,n≤1. m^2 + n^2 \le mn - m - n + 2 \Rightarrow (2m - n + 1)^2 + 3(n + 1)^2 \le 12 \Rightarrow -3 \le m, n \le 1.
m2+n2≤−(mn−m−n+2)⇒(2m+n−1)2+3(n−1/3)2≤−20/3, m^2 + n^2 \le -(mn - m - n + 2) \Rightarrow (2m + n - 1)^2 + 3(n - 1/3)^2 \le -20/3,
which is a contradiction.

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