Question 5
(GDR 2) For every integer $d\ge1$, let $M_d$ be the set of all positive integers that cannot be written as a sum of an arithmetic progression with difference $d$, having at least two terms and consisting of positive integers. Let $A=M_1$, $B=M_2\setminus\{2\}$, $C=M_3$. Prove that every $c\in C$ may be written in a unique way as $c=ab$ with $a\in A,b\in B$.