a(n) = 2*a(n-1) + 2*a(n-2) - a(n-3), with a(0)=4, a(1)=6, a(2)=20.

Terms

a(0) =4a(1) =6a(2) =20a(3) =48a(4) =130a(5) =336a(6) =884a(7) =2310a(8) =6052a(9) =15840a(10) =41474a(11) =108576a(12) =284260a(13) =744198a(14) =1948340a(15) =5100816a(16) =13354114a(17) =34961520a(18) =91530452a(19) =239629830a(20) =627359044a(21) =1642447296a(22) =4299982850a(23) =11257501248a(24) =29472520900a(25) =77160061446a(26) =202007663444a(27) =528862928880