a(n) = 2^n*(n/8 + 1) - n.

Terms

a(0) =3a(1) =8a(2) =20a(3) =47a(4) =106a(5) =233a(6) =504a(7) =1079a(8) =2294a(9) =4853a(10) =10228a(11) =21491a(12) =45042a(13) =94193a(14) =196592a(15) =409583a(16) =851950a(17) =1769453a(18) =3669996a(19) =7602155a(20) =15728618a(21) =32505833a(22) =67108840a(23) =138412007a(24) =285212646a(25) =587202533a(26) =1207959524a(27) =2483027939a(28) =5100273634a(29) =10468982753