To find all pairs of positive integers (m,n) such that mn−1 divides (n2−n+1)2, we need to analyze the given condition and derive the solutions.
First, let's denote d=mn−1. We need d to divide (n2−n+1)2. This implies:
d∣(n2−n+1)2.
We start by considering the trivial solution (m,n)=(2,2):
2⋅2−1=3and(22−2+1)2=32=9,
which satisfies 3∣9.
Next, we explore other potential solutions. Suppose (m,n) is a solution with 2<n<m. We need to find k<n such that (n,k) is also a solution. This requires:
(n2−n+1)2≡−1(modn).
We analyze the inequalities:
n2−1≤mn−1(n2−n+1)2≤n2−1(n2−n+1)2.
Simplifying, we get:
n2−1(n2−n+1)2≥n2−1⟹n2−n+1≥n2−1⟹n≤2.
This implies that the solutions are limited to specific cases. By fixing initial values a0 and b0 with b0≤2, we can generate all solutions using the recurrence relation:
(ai,bi)i=0∞whereai=bi+1,bi=bi+11[ai+1bi+1−1(bi+12−bi+1+1)2+1]∀ i∈N∗.
By working through this recurrence, we find that the solutions are:
(m,n)=(2,2)and(m,n)=(bℓ−1,bℓ), (bℓ,bℓ+1)∀ ℓ∈N∗.
Cleaning up the recurrence relation:
bi+2=bibi+1−1bi+13−2bi+12+3bi+1+bi−2∀ i∈N∗,b0=2,b1=5,
we get the sequence:
(bi)i=0∞=2,5,10,17,…,
which can be generalized as:
bi=(i+1)2+1.
Thus, the pairs (m,n) that satisfy the condition are:
(m,n)=(2,2)and(m,n)=((i+1)2+1,(i+2)2+1)∀ i∈N.
The answer is: (2, 2) and } ((i+1)^2 + 1, (i+2)^2 + 1) for all } i ∈N.