Number of stable towers of 2 X 2 LEGO blocks.

Terms

a(0) =1a(1) =3a(2) =7a(3) =19a(4) =53a(5) =149a(6) =419a(7) =1191a(8) =3403a(9) =9755a(10) =28077a(11) =81097a(12) =234861a(13) =681697a(14) =1982723a(15) =5777375a(16) =16861521a(17) =49281525a(18) =144222987a(19) =422566835a(20) =1239423303a(21) =3638872529a(22) =10693065215a(23) =31448140529a(24) =92558787745a(25) =272612601065a(26) =803448576111