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

Terms

a(0) =0a(1) =2a(2) =5a(3) =10a(4) =22a(5) =56a(6) =155a(7) =449a(8) =1325a(9) =3951a(10) =11826a(11) =35447a(12) =106308a(13) =318886a(14) =956617a(15) =2869806a(16) =8609370a(17) =25828060a(18) =77484127a(19) =232452325a(20) =697356913a(21) =2092070675a(22) =6276211958a(23) =18828635803a(24) =56485907336a(25) =169457721930a(26) =508373165709