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

Terms

a(0) =1a(1) =5a(2) =12a(3) =38a(4) =107a(5) =316a(6) =915a(7) =2671a(8) =7771a(9) =22640a(10) =65922a(11) =191993a(12) =559112a(13) =1628281a(14) =4741905a(15) =13809541a(16) =40216516a(17) =117119750a(18) =341079507a(19) =993301748a(20) =2892722267a(21) =8424270271a(22) =24533405595a(23) =71446899736a(24) =208069745986a(25) =605946785585