Expansion of (1+3x+x^2)/(1-3x+x^2).

Terms

a(0) =1a(1) =6a(2) =18a(3) =48a(4) =126a(5) =330a(6) =864a(7) =2262a(8) =5922a(9) =15504a(10) =40590a(11) =106266a(12) =278208a(13) =728358a(14) =1906866a(15) =4992240a(16) =13069854a(17) =34217322a(18) =89582112a(19) =234529014a(20) =614004930a(21) =1607485776a(22) =4208452398a(23) =11017871418a(24) =28845161856a(25) =75517614150a(26) =197707680594