dat nr natural n=p₁^a₁*p₂^a₂*....*ak^ak, unde p₁,p₂,...pk sunt numere prime
suma tuturor divizorilor lui n e data de formula
Sn= [p₁^(a₁+1)-1]/(p₁-1)* [p₂^(a₂+1)-1]/(p₂-1)*...*[pk^(ak+1)-1]/(pk-1)
aflam nr xy barat
xy= yx+63, x si y diferiti de 0
10x+y= 10y+x+63
10x-x-10y+y=63
9(x-y)= 63
x-y= 7
⇒ (x,y) ∈{(9,2), (8,1)}
pt xy=92= 2²*43 ⇒S92= (2³-1)*(43+1)= 7*44
pt xy=81= 3⁴ ⇒S81= (3⁵-1)/2= 242/2= 121
O zi buna!
