come inif n is not a power of 2, then there exists an odd prime dividing n, say p. then, n=pq. then, 2^n+1=2^pq+1=(2^q+1)(2^(p-1)q-2^(p-2)q+...+2^2q-2^q+1) and p is odd, LHS=RHS. QED.