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

Terms

a(0) =2a(1) =6a(2) =17a(3) =48a(4) =135a(5) =379a(6) =1064a(7) =2987a(8) =8385a(9) =23538a(10) =66075a(11) =185483a(12) =520680a(13) =1461631a(14) =4103029a(15) =11517850a(16) =32332423a(17) =90762215a(18) =254783864a(19) =715218523a(20) =2007731289a(21) =5636018642a(22) =15821193955a(23) =44412588755a(24) =124673147016a(25) =349977202919