MathDB
(ax + b)/(cx+d)=(Ax + B)/(Cx+D) \in Q (HOMC 2013 Q15)

Source:

August 1, 2019
algebrarational

Problem Statement

Denote by QQ and NN^* the set of all rational and positive integer numbers, respectively. Suppose that ax+bcx+dQ\frac{ax + b}{cx + d} \in Q for every xNx \in N^*: Prove that there exist integers A,B,C,DA,B,C,D such that ax+bcx+d=Ax+BCx+D\frac{ax + b}{cx + d}= \frac{Ax + B}{Cx+D} for all xNx \in N^*