Find all pairs (p,q) of prime numbers with p>q for which the number (p+q)p−q(p−q)p+q−1(p+q)p+q(p−q)p−q−1 is an integer.
Solution
Let M=(p+q)p−q(p−q)p+q−1, which is relatively prime with both p+q and p−q. Denote by (p−q)−1 the multiplicative inverse of (p−q) modulo M. By eliminating the term −1 in the numerator, (p+q)p+q(p−q)p−q−1(p+q)2q((p+q)⋅(p−q)−1)2q≡(p+q)p−q(p−q)p+q−1(modM)≡(p−q)2q(modM)≡1(modM)(1)(2)
Case 1: q⩾5. Consider an arbitrary prime divisor r of M. Notice that M is odd, so r⩾3. By (2), the multiplicative order of ((p+q)⋅(p−q)−1) modulo r is a divisor of the exponent 2q in (2), so it can be 1,2,q or 2q. By Fermat's theorem, the order divides r−1. So, if the order is q or 2q then r≡1(modq). If the order is 1 or 2 then r∣(p+q)2−(p−q)2=4pq, so r=p or r=q. The case r=p is not possible, because, by applying Fermat's theorem, M=(p+q)p−q(p−q)p+q−1≡qp−q(−q)p+q−1=(q2)p−1≡q2−1=(q+1)(q−1)(modp) and the last factors q−1 and q+1 are less than p and thus p∤M. Hence, all prime divisors of M are either q or of the form kq+1; it follows that all positive divisors of M are congruent to 0 or 1 modulo q. Now notice that M=((p+q)2p−q(p−q)2p+q−1)((p+q)2p−q(p−q)2p+q+1) is the product of two consecutive positive odd numbers; both should be congruent to 0 or 1 modulo q. But this is impossible by the assumption q⩾5. So, there is no solution in Case 1.
Case 2: q=2. By (1), we have M∣(p+q)2q−(p−q)2q=(p+2)4−(p−2)4, so (p+2)p−2(p−2)p+2−1=M⩽(p+2)4−(p−2)4⩽(p+2)4−1,(p+2)p−6(p−2)p+2⩽1. If p⩾7 then the left-hand side is obviously greater than 1. For p=5 we have (p+2)p−6(p−2)p+2=7−1⋅37 which is also too large. There remains only one candidate, p=3, which provides a solution: (p+q)p−q(p−q)p+q−1(p+q)p+q(p−q)p−q−1=51⋅15−155⋅11−1=43124=781. So in Case 2 the only solution is (p,q)=(3,2).
Case 3: q=3. Similarly to Case 2, we have M(p+q)2q−(p−q)2q=64⋅((2p+3)6−(2p−3)6). Since M is odd, we conclude that M(2p+3)6−(2p−3)6 and (p+3)p−3(p−3)p+3−1=M⩽(2p+3)6−(2p−3)6⩽(2p+3)6−1,64(p+3)p−9(p−3)p+3⩽1. If p⩾11 then the left-hand side is obviously greater than 1. If p=7 then the left-hand side is 64⋅10−2⋅410>1. If p=5 then the left-hand side is 64⋅8−4⋅28=22>1. Therefore, there is no solution in Case 3.
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