a(n) = 3^(n-1) - lcm{1..n}.

Terms

a(0) =0a(1) =1a(2) =3a(3) =15a(4) =21a(5) =183a(6) =309a(7) =1347a(8) =4041a(9) =17163a(10) =31329a(11) =149427a(12) =171081a(13) =1233963a(14) =4422609a(15) =13628187a(16) =30794481a(17) =116887923a(18) =154627929a(19) =929468907a(20) =3253991841a(21) =10227560643a(22) =26026830729a(23) =88788949947a(24) =255658392081a(25) =820517465043