a) n=2^(k-1) satifies the equation σ(n)=2n-1

b) if 2^k - 1 is prime, then n=2^(k-1)(2^k - 1) satifies the equation
σ(n)=2n

c) If 2^k - 3 is prime, then n= 2^(k-1)(2^k - 3) satisfies the equation
σ(n)=2n+2