a(n) = 20-n.

Terms

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