No other terms below 2^64.
Terms a(2) and a(3) are of the form (4^k-1)/3=A002450(k). Terms A002450(k) belong to this sequence for k in A303009. Correspondingly, a(4) <= A002450(A303009(3)) = (4^41-1)/3 = 1611901092819505566274901.
Table of n, a(n) for n=1..3.
Numbers 2*a(n)+1 are listed in A303448.
Subsequence of A006970.
Cf. A298758, A300193.
Sequence in context: A034606 A233839 A059044 * A107617 A234980 A179737
Adjacent sequences: A303444 A303445 A303446 * A303448 A303449 A303450
Max Alekseyev, Apr 24 2018
a(1) from Amiram Eldar, Jan 26 2018