Coefficient of x^floor(n/2) in the expansion of (1+x+x^3)^n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =6a(5) =10a(6) =26a(7) =42a(8) =126a(9) =198a(10) =612a(11) =957a(12) =2970a(13) =4654a(14) =14534a(15) =22815a(16) =71838a(17) =112846a(18) =357884a(19) =562343a(20) =1793296a(21) =2818536a(22) =9026976a(23) =14191575a(24) =45612450a(25) =71726650a(26) =231224060a(27) =363685545a(28) =1175422590a(29) =1849145850