a(n) = A121880(2*n)/2.

Terms

a(0) =1a(1) =5a(2) =22a(3) =91a(4) =369a(5) =1486a(6) =5962a(7) =23884a(8) =95607a(9) =382568a(10) =1530552a(11) =6122765a(12) =24492171a(13) =97970902a(14) =391888040a(15) =1567561019a(16) =6270261786a(17) =25081082556a(18) =100324401036a(19) =401297745749