If one of the numbers is 2, then p=q=2, and we can assume p,q≥3. Since (p,p2+8)=(q,q2+8)=1, we have: q∣p2+8 and p∣q2+8⇒pq∣(p2+8)(q2+8)⇒pq∣8(p2+q2+8)⇒pq∣(p2+q2+8).
For a fixed k∈N∗, we determine the solutions in N∗×N∗ of the equation p2+q2+8=kpq with p,q<2023.
Let's assume that (p0,q0) is a solution for which the sum p+q is minimal and p0≥q0. If p0=q0, since p02∣8, we have p0=q0=1 or p0=q0=2 (cases we will analyze later). Now, let's assume that for p0≥3, we have p0>q0.
If p0≥5 and p′ is the second solution of the equation x2−(q0k)x+q02+8=0, then we have p′=p0q02+8≤p0p02−2p0+9<p0. Since p′+q0<p0+q0, it follows that p0∈{3,4}.
If p0=3, then q0∣17, so q0=1 and k=6. Using Vieta jumping, we obtain the solutions given by the sequence p0=3,q0=1,qn+1=pn,pn+1=6pn−qn. Thus, we obtain the solutions (3,1), (17,3), (99,17), (577,99), while the remaining solutions have p≥2023. The only solution remaining is (17,3).
If p0=4, then q0∣24 and q0≤3, so q0 is 2 or 1, but both cases are impossible.
Now, let's consider the cases p0=2 and p0=1.
If p0=2, then q0∣12 and q0≤2, so q0=2 (the case q0=1 is not possible) and k=4. Using Vieta jumping, all solutions have both components even. The acceptable solution is (2,2).