A modified Heron sequence starting from 1,3.

Terms

a(0) =1a(1) =3a(2) =12a(3) =49a(4) =108a(5) =243a(6) =624a(7) =1681a(8) =5616a(9) =47628a(10) =49300a(11) =27361200a(12) =29949750a(13) =920048500a(14) =1054364310a(15) =1278524208a(16) =9992125972