a(n) = floor(sqrt(2*5^n)).

Terms

a(0) =1a(1) =3a(2) =7a(3) =15a(4) =35a(5) =79a(6) =176a(7) =395a(8) =883a(9) =1976a(10) =4419a(11) =9882a(12) =22097a(13) =49410a(14) =110485a(15) =247052a(16) =552427a(17) =1235264a(18) =2762135a(19) =6176323a(20) =13810679a(21) =30881617a(22) =69053396a(23) =154408088a(24) =345266983a(25) =772040444a(26) =1726334915a(27) =3860202221a(28) =8631674575a(29) =19301011109