a(n) = 18-n.

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