a(n) = ((3+sqrt(5))*(1+sqrt(5))^n + (3-sqrt(5))*(1-sqrt(5))^n)/2.

Terms

a(0) =3a(1) =8a(2) =28a(3) =88a(4) =288a(5) =928a(6) =3008a(7) =9728a(8) =31488a(9) =101888a(10) =329728a(11) =1067008a(12) =3452928a(13) =11173888a(14) =36159488a(15) =117014528a(16) =378667008a(17) =1225392128a(18) =3965452288a(19) =12832473088a(20) =41526755328a(21) =134383403008a(22) =434873827328