a(n) = (6/n^3) * Sum_{d|n} moebius(n/d)*binomial(2*d,d).

Terms

a(0) =12a(1) =3a(2) =4a(3) =6a(4) =12a(5) =25a(6) =60a(7) =150a(8) =400a(9) =1107a(10) =3180a(11) =9386a(12) =28404a(13) =87711a(14) =275764a(15) =880470a(16) =2849916a(17) =9336508a(18) =30918732a(19) =103384758a(20) =348725540a(21) =1185630123a(22) =4060210764a(23) =13996354586a(24) =48541672872a(25) =169293988125a(26) =593488622344