a(n) = 3^n - lcm{1..n}, with a(0) = 0.

Terms

a(0) =0a(1) =2a(2) =7a(3) =21a(4) =69a(5) =183a(6) =669a(7) =1767a(8) =5721a(9) =17163a(10) =56529a(11) =149427a(12) =503721a(13) =1233963a(14) =4422609a(15) =13988547a(16) =42326001a(17) =116887923a(18) =375168249a(19) =929468907a(20) =3253991841a(21) =10227560643a(22) =31148267049a(23) =88788949947a(24) =277075307601a(25) =820517465043