Second trisection of A246584.

Terms

a(0) =6a(1) =48a(2) =282a(3) =1260a(4) =4896a(5) =16836a(6) =53232a(7) =156384a(8) =433776a(9) =1143696a(10) =2890266a(11) =7032576a(12) =16557084a(13) =37838052a(14) =84206724a(15) =182913216a(16) =388685430a(17) =809399280a(18) =1654446816a(19) =3323927340a(20) =6572070528a(21) =12801615744a(22) =24590359284a(23) =46619988384a(24) =87302773392a(25) =161597518272a(26) =295849759728