Partial sums of round(4^n/9).

Terms

a(0) =0a(1) =0a(2) =2a(3) =9a(4) =37a(5) =151a(6) =606a(7) =2426a(8) =9708a(9) =38835a(10) =155343a(11) =621377a(12) =2485512a(13) =9942052a(14) =39768214a(15) =159072861a(16) =636291449a(17) =2545165803a(18) =10180663218a(19) =40722652878a(20) =162890611520a(21) =651562446087