Second diagonal of A027537.

Terms

a(0) =3a(1) =16a(2) =55a(3) =156a(4) =399a(5) =960a(6) =2223a(7) =5020a(8) =11143a(9) =24432a(10) =53079a(11) =114492a(12) =245535a(13) =524032a(14) =1113823a(15) =2358972a(16) =4980375a(17) =10485360a(18) =22019655a(19) =46136860a(20) =96468463a(21) =201326016a(22) =419429775a(23) =872414556a(24) =1811938599a(25) =3758095600a(26) =7784627383a(27) =16106126460a(28) =33285995583a(29) =68719475712