This paper develops a complete theory of linear Diophantine systems N = pA + qB for coprime pairs (p, q) satisfying p ≡ 1 (mod q). The central result establishes that the minimal coefficient A₀ equals the generalised digital root drq (N), with a closed formula for the representation count R (p, q;N) and arithmetic progressions.
Bilal El Issaoui (Fri,) studied this question.