a(n) = n-20.

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