a(n) = floor(sqrt(2*7^n)).

Terms

a(0) =1a(1) =3a(2) =9a(3) =26a(4) =69a(5) =183a(6) =485a(7) =1283a(8) =3395a(9) =8983a(10) =23768a(11) =62886a(12) =166380a(13) =440202a(14) =1164665a(15) =3081415a(16) =8152659a(17) =21569910a(18) =57068618a(19) =150989371a(20) =399480328a(21) =1056925602a(22) =2796362297a(23) =7398479214a(24) =19574536080a(25) =51789354498a(26) =137021752562a(27) =362525481486a(28) =959152267937