a(n)=2^(2^n)*sum(k=0,n,1/2^(2^k)).A085010a(n)=2^(2^n)*sum(k=0,n,1/2^(2^k)).Terms a(0) =1a(1) =3a(2) =13a(3) =209a(4) =53505a(5) =3506503681External referencesoeis: A085010