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

Terms

a(0) =0a(1) =0a(2) =1a(3) =8a(4) =36a(5) =149a(6) =604a(7) =2424a(8) =9705a(9) =38832a(10) =155340a(11) =621373a(12) =2485508a(13) =9942048a(14) =39768209a(15) =159072856a(16) =636291444a(17) =2545165797a(18) =10180663212a(19) =40722652872a(20) =162890611513a(21) =651562446080