a(n) = n-18.

Terms

a(0) =-18a(1) =-17a(2) =-16a(3) =-15a(4) =-14a(5) =-13a(6) =-12a(7) =-11a(8) =-10a(9) =-9a(10) =-8a(11) =-7a(12) =-6a(13) =-5a(14) =-4a(15) =-3a(16) =-2a(17) =-1a(18) =0a(19) =1a(20) =2a(21) =3a(22) =4a(23) =5a(24) =6a(25) =7a(26) =8a(27) =9a(28) =10a(29) =11