a(n) = 2*a(n-1) + a(n-2) + 3*a(n-3), a(0)=1.

Terms

a(0) =1a(1) =2a(2) =5a(3) =15a(4) =41a(5) =112a(6) =310a(7) =855a(8) =2356a(9) =6497a(10) =17915a(11) =49395a(12) =136196a(13) =375532a(14) =1035445a(15) =2855010a(16) =7872061a(17) =21705467a(18) =59848025a(19) =165017700a(20) =454999826a(21) =1254561427a(22) =3459175780a(23) =9537912465a(24) =26298684991a(25) =72512809787a(26) =199938041960a(27) =551284948680