a(n) = C(2n-1,n) + C(2n+1,n+1) - C(0,n).

Terms

a(0) =1a(1) =4a(2) =13a(3) =45a(4) =161a(5) =588a(6) =2178a(7) =8151a(8) =30745a(9) =116688a(10) =445094a(11) =1704794a(12) =6552378a(13) =25258600a(14) =97617060a(15) =378098955a(16) =1467343305a(17) =5704370760a(18) =22210199550a(19) =86595896310a(20) =338052201630