Row 2 of rectangular table A124460.

Terms

a(0) =1a(1) =3a(2) =8a(3) =23a(4) =73a(5) =251a(6) =919a(7) =3549a(8) =14371a(9) =60720a(10) =266481a(11) =1209807a(12) =5662008a(13) =27238884a(14) =134391046a(15) =678739990a(16) =3503708942a(17) =18462855900a(18) =99211177417a(19) =543161148837