a(1)=1; a(n) = floor((3 + sqrt(21))*a(n-1)/2) for n > 1.

Terms

a(0) =1a(1) =3a(2) =11a(3) =41a(4) =155a(5) =587a(6) =2225a(7) =8435a(8) =31979a(9) =121241a(10) =459659a(11) =1742699a(12) =6607073a(13) =25049315a(14) =94969163a(15) =360055433a(16) =1365073787a(17) =5175387659a(18) =19621384337a(19) =74390315987a(20) =282035100971