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

Terms

a(0) =6a(1) =3a(2) =6a(3) =15a(4) =48a(5) =171a(6) =678a(7) =2871a(8) =12858a(9) =60084a(10) =290814a(11) =1448679a(12) =7394106a(13) =38527779a(14) =204365880a(15) =1101000087a(16) =6013054788a(17) =33239486925a(18) =185736687366