a(n) = floor((1+sqrt(3))^n).

Terms

a(0) =1a(1) =2a(2) =7a(3) =20a(4) =55a(5) =152a(6) =415a(7) =1136a(8) =3103a(9) =8480a(10) =23167a(11) =63296a(12) =172927a(13) =472448a(14) =1290751a(15) =3526400a(16) =9634303a(17) =26321408a(18) =71911423a(19) =196465664a(20) =536754175a(21) =1466439680a(22) =4006387711a(23) =10945654784a(24) =29904084991a(25) =81699479552a(26) =223207129087