Multinomial coefficient n!/([n/3]![(n+1)/3]![(n+2)/3]!).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =12a(5) =30a(6) =90a(7) =210a(8) =560a(9) =1680a(10) =4200a(11) =11550a(12) =34650a(13) =90090a(14) =252252a(15) =756756a(16) =2018016a(17) =5717712a(18) =17153136a(19) =46558512a(20) =133024320a(21) =399072960a(22) =1097450640a(23) =3155170590a(24) =9465511770a(25) =26293088250a(26) =75957810500a(27) =227873431500a(28) =638045608200