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

Terms

a(0) =2a(1) =7a(2) =22a(3) =72a(4) =232a(5) =752a(6) =2432a(7) =7872a(8) =25472a(9) =82432a(10) =266752a(11) =863232a(12) =2793472a(13) =9039872a(14) =29253632a(15) =94666752a(16) =306348032a(17) =991363072a(18) =3208118272a(19) =10381688832a(20) =33595850752a(21) =108718456832a(22) =351820316672