a(n) = A055450(n, n-5).

Terms

a(0) =1a(1) =6a(2) =20a(3) =48a(4) =90a(5) =132a(6) =561a(7) =2420a(8) =10571a(9) =46672a(10) =207963a(11) =934064a(12) =4224685a(13) =19225588a(14) =87969426a(15) =404479884a(16) =1867924322a(17) =8660317820a(18) =40295911480a(19) =188105782260a(20) =880716750140