A trisection of A161804: a(n) = A161804(3n) for n>=0.

Terms

a(0) =1a(1) =12a(2) =66a(3) =255a(4) =903a(5) =2970a(6) =8571a(7) =23001a(8) =59763a(9) =145164a(10) =333693a(11) =751059a(12) =1623009a(13) =3363576a(14) =6872307a(15) =13677228a(16) =26351985a(17) =50309910a(18) =94392525a(19) =172538934a(20) =313558506a(21) =563064207a(22) =988996095a(23) =1730456433a(24) =3001805067a(25) =5106353439