U(N)=U(2007^2000)=U(7^2000)
U(7^1)=7
U(7^2)=9
U(7^3)=3
U(7^4)=1
U(7^5)=7
.................
2000:4=500 (rest 0)
Am impartit la 4 ptr ca U(7^n) poate fi 7, 9, 3 sau 1
U(7^2000)=U(7^{4×500+0} )=U(7^4)=1
R: U(N)=1
N=2007^2000=2007^(2·1000)=(2007^1000)^2 => e patrat perfect
Nota: " ^ " inseamna " la puterea..."
