a(n) = 21-n.

Terms

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