a(n) = f(1,n,n), where f is the Sudan function defined in A260002.

Terms

a(0) =0a(1) =3a(2) =12a(3) =35a(4) =90a(5) =217a(6) =504a(7) =1143a(8) =2550a(9) =5621a(10) =12276a(11) =26611a(12) =57330a(13) =122865a(14) =262128a(15) =557039a(16) =1179630a(17) =2490349a(18) =5242860a(19) =11010027a(20) =23068650a(21) =48234473a(22) =100663272a(23) =209715175a(24) =436207590a(25) =905969637a(26) =1879048164a(27) =3892314083a(28) =8053063650a(29) =16642998241