a(n) = A027113(n, 2n-2).

Terms

a(0) =2a(1) =7a(2) =20a(3) =56a(4) =155a(5) =429a(6) =1188a(7) =3298a(8) =9181a(9) =25634a(10) =71781a(11) =201569a(12) =567538a(13) =1601971a(14) =4532460a(15) =12851882a(16) =36516613a(17) =103955432a(18) =296471643a(19) =846932919a(20) =2423256440a(21) =6943726380a(22) =19924638557a(23) =57247669927a(24) =164688166948a(25) =474323305547