Powers of sqrt(15) rounded down.

Terms

a(0) =1a(1) =3a(2) =15a(3) =58a(4) =225a(5) =871a(6) =3375a(7) =13071a(8) =50625a(9) =196069a(10) =759375a(11) =2941046a(12) =11390625a(13) =44115700a(14) =170859375a(15) =661735513a(16) =2562890625a(17) =9926032708a(18) =38443359375a(19) =148890490631a(20) =576650390625