a(n)=4a(n-1)-4a(n-2)+2a(n-3).

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =28a(5) =80a(6) =228a(7) =648a(8) =1840a(9) =5224a(10) =14832a(11) =42112a(12) =119568a(13) =339488a(14) =963904a(15) =2736800a(16) =7770560a(17) =22062848a(18) =62642752a(19) =177860736a(20) =504997632a(21) =1433833088a(22) =4071063296a(23) =11558916096a(24) =32819077376a(25) =93182771712a(26) =264572609536