a(n) = 20 + floor( (1 + Sum_{j=1..n-1} a(j)) / 2 ).

Terms

a(0) =20a(1) =30a(2) =45a(3) =68a(4) =102a(5) =153a(6) =229a(7) =344a(8) =516a(9) =774a(10) =1161a(11) =1741a(12) =2612a(13) =3918a(14) =5877a(15) =8815a(16) =13223a(17) =19834a(18) =29751a(19) =44627a(20) =66940a(21) =100410a(22) =150615a(23) =225923a(24) =338884a(25) =508326a(26) =762489a(27) =1143734a(28) =1715601a(29) =2573401