a(n) = 19 - n.

Terms

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