Upper (1/5,1/2) midsequence of (3^n) and (5*n); see Comments.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =23a(5) =57a(6) =156a(7) =450a(8) =1326a(9) =3952a(10) =11827a(11) =35448a(12) =106309a(13) =318887a(14) =956618a(15) =2869807a(16) =8609371a(17) =25828061a(18) =77484128a(19) =232452326a(20) =697356914a(21) =2092070676a(22) =6276211959a(23) =18828635804a(24) =56485907337a(25) =169457721931a(26) =508373165710