a(n) = T(n,n+4), T given by A027023.

Terms

a(0) =1a(1) =25a(2) =85a(3) =215a(4) =477a(5) =985a(6) =1949a(7) =3755a(8) =7113a(9) =13329a(10) =24805a(11) =45959a(12) =84917a(13) =156625a(14) =288573a(15) =531323a(16) =977873a(17) =1799273a(18) =3310133a(19) =6089111a(20) =11200525a(21) =20601961a(22) =37893981a(23) =69699051a(24) =128197785a(25) =235793825a(26) =433693893a(27) =797688967a(28) =1467180389