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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =15a(5) =38a(6) =97a(7) =247a(8) =629a(9) =1602a(10) =4080a(11) =10391a(12) =26464a(13) =67399a(14) =171653a(15) =437169a(16) =1113390a(17) =2835602a(18) =7221763a(19) =18392518a(20) =46842401a(21) =119299083a(22) =303833085a(23) =773807654a(24) =1970747476a(25) =5019135691a(26) =12782826512a(27) =32555536191a(28) =82913034585