G.f.: 1/((1-x)*(1-x^2))^6.

Terms

a(0) =1a(1) =6a(2) =27a(3) =92a(4) =273a(5) =714a(6) =1715a(7) =3816a(8) =8007a(9) =15938a(10) =30381a(11) =55692a(12) =98735a(13) =169806a(14) =284349a(15) =464672a(16) =742950a(17) =1164228a(18) =1791426a(19) =2710344a(20) =4037670a(21) =5928988a(22) =8591154a(23) =12294672a(24) =17392258a(25) =24337404a(26) =33711510a(27) =46251016a(28) =62886162a(29) =84779748