a(0)=0, a(1)=1, a(n) = min{3 a(k) + (3^(n-k)-1)/2, k=0..(n-1)} for n>=2.

Terms

a(0) =0a(1) =1a(2) =4a(3) =7a(4) =16a(5) =25a(6) =34a(7) =61a(8) =88a(9) =115a(10) =142a(11) =223a(12) =304a(13) =385a(14) =466a(15) =547a(16) =790a(17) =1033a(18) =1276a(19) =1519a(20) =1762a(21) =2005a(22) =2734a(23) =3463a(24) =4192a(25) =4921a(26) =5650a(27) =6379a(28) =7108a(29) =9295