A024957 a(n) = Sum(a(2i-1)*a(n-2i+1), i = 1,2,...,[ (n+2)/4 ]).

2, 1, 1, 2, 4, 9, 20, 44, 97, 230, 540, 1257, 2920, 7168, 17393, 41806, 100408, 250098, 618124, 1517008, 3716627, 9375710, 23479016, 58419236, 145210076, 368786352, 931600246, 2341625080, 5877503872, 15027036202, 38225273720, 96794415372