For n>3, a(n) = 7*n - 21 + 2*(-1)^n.

Terms

a(0) =1a(1) =3a(2) =4a(3) =9a(4) =12a(5) =23a(6) =26a(7) =37a(8) =40a(9) =51a(10) =54a(11) =65a(12) =68a(13) =79a(14) =82a(15) =93a(16) =96a(17) =107a(18) =110a(19) =121a(20) =124a(21) =135a(22) =138a(23) =149a(24) =152a(25) =163a(26) =166a(27) =177a(28) =180a(29) =191