Twice A137829.

Terms

a(0) =2a(1) =4a(2) =12a(3) =24a(4) =50a(5) =92a(6) =172a(7) =296a(8) =510a(9) =840a(10) =1372a(11) =2176a(12) =3424a(13) =5268a(14) =8040a(15) =12072a(16) =17976a(17) =26428a(18) =38564a(19) =55680a(20) =79846a(21) =113500a(22) =160320a(23) =224768a(24) =313320a(25) =433916a(26) =597788a(27) =818840a(28) =1116238a(29) =1513900