a(0)=0, a(1)=1, a(n) = min{4 a(k) + (4^(n-k)-1)/3, k=0..(n-1)} for n>=2.

Terms

a(0) =0a(1) =1a(2) =5a(3) =9a(4) =25a(5) =41a(6) =57a(7) =121a(8) =185a(9) =249a(10) =313a(11) =569a(12) =825a(13) =1081a(14) =1337a(15) =1593a(16) =2617a(17) =3641a(18) =4665a(19) =5689a(20) =6713a(21) =7737a(22) =11833a(23) =15929a(24) =20025a(25) =24121a(26) =28217a(27) =32313a(28) =36409a(29) =52793