763763/7=109109
109109/11=9919
9919/13=763
On passe de abc à abcabc en multipliant par 1001,
car abcabc=100000a+10000b+1000c+100a+10b+c=1000(100a+10b+c)+(100a+10b+c)
et abc = 100a+10b+c
Or 1001=7x11x13.
Donc abcabc = 7x11x13(abc).