a : 6=2xr rest r
a= 6x2xr + r
a=12xr + r
a=13xr
r=1 a=13x1=13 13:6=2 rest1
r=2 a=13x2=26 26:6= 4 rest2
r=3 a= 13x3=39 39:6= 6 rest 3
r=4 a=13x4=52 52:6 = 8 rest 4
r=5 a=13x5=65 65: 6= 10 rest 5
si atat k restul nu poate fi egal sau mai mare decat impartitorul (care aici e 6)
Deci nr nat sunt 13,26,39,52,65