a(n) = Sum_{k=0..n} binomial(3*n,k).

Terms

a(0) =1a(1) =4a(2) =22a(3) =130a(4) =794a(5) =4944a(6) =31180a(7) =198440a(8) =1271626a(9) =8192524a(10) =53009102a(11) =344212906a(12) =2241812648a(13) =14637774688a(14) =95786202688a(15) =628002401520