a(n) = S(n)*S(n-1), where S(n) are the generalized tribonacci numbers A001644.

Terms

a(0) =-3a(1) =3a(2) =3a(3) =21a(4) =77a(5) =231a(6) =819a(7) =2769a(8) =9301a(9) =31571a(10) =106763a(11) =361045a(12) =1221685a(13) =4132743a(14) =13980747a(15) =47297217a(16) =160004685a(17) =541291715a(18) =1831178355a(19) =6194830005a(20) =20956959933a(21) =70896891079a(22) =239842458947a(23) =811381229009