a(n) = (A135318(3*n) + A135318(3*n+1) + A135318(3*n+2))/3.

Terms

a(0) =1a(1) =3a(2) =8a(3) =23a(4) =64a(5) =185a(6) =512a(7) =1479a(8) =4096a(9) =11833a(10) =32768a(11) =94663a(12) =262144a(13) =757305a(14) =2097152a(15) =6058439a(16) =16777216a(17) =48467513a(18) =134217728a(19) =387740103a(20) =1073741824a(21) =3101920825a(22) =8589934592a(23) =24815366599a(24) =68719476736a(25) =198522932793a(26) =549755813888